When originally announced in passing at E3 2011, the game's development was slated to begin sometime after October 2011, once development on Kid Icarus: Uprising had completed.  In February 2012, it was then confirmed that development on the fourth game had begun. The game was unmentioned during E3 2012, something which many fans were disappointed about despite the known extremely early state of the game; the "first step of the process" was taken shortly after in mid-June. Masahiro Sakurai has expressed disappointment that fans will be waiting for longer than expected for the game to be released due to the earliness of the initial announcement. Shortly afterward, it was revealed during a Nintendo Direct that Namco Bandai was the primary developer alongside Sora Ltd., and had already completed a working prototype.
The paired versions of the game were officially revealed at E3 2013 in the form of a trailer on June 11, 2013.
The 3DS version is scheduled to launch in Japan on September 13, 2014 and in most other parts of the world on October 3, 2014. The 3DS version will be released in stores one day earlier in Germany on October 2 in order to avoid coinciding with a national holiday, and one day later in Australia on October 4. The Wii U version will be released in the holiday/winter season of 2014.
Sakurai has stated, in response to a fan asking him about whether a "child Link" would appear in the game, that he had not at the time decided on who would appear in the game. However, he had also said that he "can't say that it's entirely out of the realm of possibility that some Capcom character could appear in the next Smash Bros." Indeed, Capcom's Mega Man was ultimately confirmed as playable.
Sakurai has revealed that one feature of the 3DS Smash Bros. title will be that players can improve their character through battles and rewards, then transfer them to the Wii U Smash title to play against friends. He said the 3DS title is intended to offer a new experience for veteran Smash Bros. fans, and that neither the 3DS game nor the Wii U game would simply be sequels like Melee and Brawl were, and that they'd do more than just add characters and stages. It has also been revealed that they are looking towards co-operative play for the Wii U title. Official Nintendo Magazine said "there is merit in having skilled and unskilled players play together, so one emphasis will be on elements of players helping one-another". They also stated that the graphics would be significantly stepped up, as the Wii U can handle high quality graphics, dynamic effects and smooth character movements in HD at 60 frames per second. Sakurai is also quoted as claiming that the new game is unlikely to emphasize new playable characters, focusing instead on gameplay balance and distinctiveness of its characters. 
The trailer shown at the E3 2013 Nintendo Direct confirmed several different characters. Since that time, other characters have been revealed either on Sakurai's "Pic of the day" posts in the Director's Room, or in trailers shown as part of Nintendo Directs or on the game's website.
Shortly after the initial E3 2013 trailers, Sakurai said there would be a single-player story mode included in the new Smash Bros., but that it would be different from Brawl's Subspace Emissary, and that there would be no cutscenes. However, he would later recant this, and announce that he decided to cut any sort of story mode altogether , while stating the 3DS version would have a different sort of separate mode from standard versus play. He did not provide any other details as to what this, or any other single-player modes will consist of. On 20 January 2014, on a Miiverse post, Sakurai repeated the fact the game would not have a story mode. Screenshots that people thought were from a new story mode in the 3DS version were later revealed to be from Smash Run, which also seems to be the new 3DS mode Sakurai implied when he announced the cutting of the story mode.
Outside of what little info is available on Smash Run, All-Star Mode was confirmed to return in the April Nintendo Direct. Outside of these modes, details about the games' other single-player modes are still unknown at this time.
Smash Run (3DS exclusive): In this mode, up to 4 players have 5 minutes to traverse a huge dungeon-like enviroment, collecting various power-ups and facing many enemies from various games. After the time limit, the players go at it in a battle utilizing their boosted powers, and can then do subsequent matches with those power-ups. Sakurai mentioned in the Smash Bros. Direct video that this mode was inspired by Kirby Air Ride's City Trial mode, which has a similar premise.
Players will also be able to have items set to their characters, in a way that may reflect Brawl's Subspace Emissary mode.
Apparent changes from Brawl
Brawl character size comparison.
SSB4 character size comparison.
The size differences between bigger and smaller characters will be somewhat more drastic, which can be seen when comparing the pictures on the right, with Sakurai remarking "A group shot of the big guys. How much bigger are they compared to Mario?" for the latter image. It should be noted that character sizes seem to fluctuate between images, suggesting that their sizes will be different in the final build.
It appears the movesets and animations of returning characters will go through a larger amount of change than the returning characters did from the transition of Melee to Brawl, with many returning characters being shown with completely new moves, and some appear to be getting significantly altered (such as Bowser).
It additionally appears there is a greater emphasis on buffing the returning characters, with even characters that were top and high tier in Brawl, such as Diddy Kong and Lucario, being shown to receive explicit significant buffs.
Mid-match character changes, such as Zelda/Sheik's Transform, will no longer exist. This means Zelda and Sheik, along with Samus and Zero Suit Samus, will be completely separate characters that are no longer able to transform to the other. This also means the Pokémon Trainer will not return as a playable character without being completely revamped as a single character.
In an interview with Kotaku, Sakurai confirmed that tripping would not return. It seems he was referring to random tripping, since forced tripping appears to have returned, as shown by the return of the banana peel item.
Similarly, it appears potential randomness in moves is being cut down; for example, Olimar now plucks his Pikmin in a set order instead of the Pikmin plucked being chosen at random, and King Dedede's Waddle Dee Throw has been replaced with a new move that involves throwing only Gordos instead of throwing one randomly chosen minion.
To appeal to both casual and hardcore gamers, Sakurai said the game's speed will be between that of Melee and Brawl. In support of this, in the gameplay footage shown so far, characters seem to be falling faster than they do in Brawl.
In a similar appeal to both casual and hardcore gamers, there are now two distinct type of With AnyoneWi-Fi modes; one called For Fun, which seems similar to the previous game's casual-styled Basic Brawl, and one called For Glory, a mode explicitly catered to competitive-styled play.
Additionally, almost every stage now will have a "Final Destination form", where the stage's design is altered into a single large flat platform with no additional platforms, and these stages cannot be played on in the For Fun mode while being the only stages available in the For Glory mode.
In a demo match between Mario and Mega Man, there appears to be no stale-move negation. It is unknown whether it has been explicitly removed, yet to be programmed at the time of the demo, or temporarily disabled for the demo.
Neutral attacks that previously ended in an indefinite number of weak hits (such as Fox's and Kirby's) will now "always transition into a finishing move", making such moves safer to use (as simply ending the move no longer leaves the opponent nearby). It is currently unclear if rapid jabs can still be held infinitely, or if they will automatically transition into the finishing hit after a period of time.
Holding the right control stick (C-Stick on GameCube controller) in a direction will now charge smash attacks until the stick is released.
The ability to act out of hitstun has been removed.
The mechanics of grabbing ledges will undergo an unprecedented overhaul. The difference between fast and slow edge options based on current damage will be removed, the length of intangibility given by grabbing the ledge will be affected by air time and current damage, and there appears to be some way to steal the ledge from opponents edge hogging, though no information about it was given.
Tether recoveries will be buffed, with their reach being improved. In addition, multiple tethers can grab onto the same edge at once, making such moves less suspectible to edgehogging, though "who actually grabs hold is determined by the same rules as the standard ledge grab".
Star KOs no longer occur near the end of a timed match; it is unclear from the relevant post's wording whether this means that all top-side KOs will be Screen KOs or typical blast KOs.
The special moves for characters can be modified using a new moveset customization feature. The customization will go beyond simple damage and knockback altering, with the moves' functions and aesthetics being able to be completely revamped.
The Wii U version will be compatible with a set of amiibo figurines utilizing the Wii U GamePad and near field communication (NFC). By using their respective figurine, players can give a character custom moves and level them up to level 50.
In general, the game is much more stylized and visually intense than previous entries, with the overall colors being bolder and brighter and many elements having been redone to stand out more.
The characters' design styles are more distinct from each other and more in-line with their home series, in contrast to Brawl which gave the characters a more unified realistic look. For example, characters from cartoony franchises such as the Mario and Kirby series are much closer to their native styles, while those from more realistic-looking franchises such as The Legend of Zelda and Metroid series maintain more realistic appearances.
In the Wii U version, most characters will always stand facing the screen regardless of which direction they face, with the intent of having them face the screen more often.
This applies to movement and attack animations as well, often involving them mirroring their stance and attacks. Due to this, there is a possibility that some attacks may behave slightly differently when they interact with each other from different directions. For example, if such an "ambidextrous" character and another character that does not use this feature attack each other, the attacks may clang in a different location or even connect with the opponent differently depending on what direction the two characters are facing.
Damage percentage is now displayed with a metallic gradient and rises through yellow shades before turning red.
Several visual effects are significantly bolder and brighter (and so are arguably more visually disruptive):
Attacks' visual effects (such as motion blurs) are in general more pronounced, with bright saturated blurs replacing the previous game's subtle ones.
Smoke trails of hit characters are now thick trails of light, colored according to which player would be credited with a potential KO. Should the KO occur, the attacker will flash with an aura of their own color.
The design of KOs themselves closely resembles those in Brawl, but the soundbite sounds much more like an actual explosion now.
Hits that deal knockback above a certain value produce a red-and-black lightning effect on contact.
Certain items such as the Bumper and Home-Run Bat have been given new, more striking designs. The Home-Run Bat, for example, is more ornate and is now colored to match the new high-knockback lightning effect.
Characters now have team colored outlines in Team Battles, and can select a color normally.
Revival platforms now have a section that changes color based on the remaining time left before the platform disappears.
Characters with high damage percentages now emit steam, presumably to make their vulnerability more clear.
In the 3DS version, players can tap on a characters icon on the bottom screen to place a marker on that character on the top screen, in order to more easily follow their movements.
Victory scenes seem to be a cross between Brawl and Melee. Like Melee, only the winner is shown in the main area, while the others are seen applauding in small windows on the screen. However, instead of a featureless black screen, the winner's area is an environment similar to that in Brawl’s victory screen.
Screen KOs now feature characters hitting the screen and staying there ("splatting") for a moment before sliding off, whereas in preceding games, they simply bumped into the screen and kept falling at their initial speed.
The Stock Icons for Stock Matches are now akin to those in Melee, in which they appeared as the character's head, as opposed to Brawl, where they were merely small circles colored depending on the player. Additionally, the stock icons are placed below the HUD that contains the character's damage percentage rather than on it.
Characters no longer "halt" as they run off of ledges, and instead have animations that transition them into their air speed.
Other apparent changes
Sakurai stated that the short movies for introducing new characters, including Mega Man, Villager, and Wii Fit Trainer, will be viewable without having to be unlocked, much like the reveal trailers for Snake and Sonic in Brawl'sMovies menu.
The Wii U and 3DS versions of the game were shown to have distinct art styles from one another in that the 3DS version uses flatter shading and optional black outlines to make characters easier to see at a distance; these outlines will be customizable as Sakurai stated players can change the size of the outlines or get rid of them completely. It is also known that several if not most stages will be version-specific, with the Wii U version's stages being based on console games, while the 3DS version's stages being based on handheld games. It is confirmed that there will be no cross-platform gameplay between the Wii U and 3DS versions due to the exclusive stages to each version, but it has been mentioned that "customized characters" from the 3DS version can be imported into the Wii U version. Very little detail has been given as to what "character customization" entails; it has only been revealed that customizations will not be different costumes and will not have to be unlocked through repetitive tasks.
While the two versions will have the same playable cast, it has been confirmed by Sakurai that there will be a few differences with the characters summoned from Assist Trophies and Pokémon summoned from Poké Balls in the 3DS version, though it has not been stated if there will either be explicit version exclusives or a few will be cut out of the 3DS version for space issues.
Another difference confirmed by Sakurai is that the trophies will be different in the two versions, with the trophies in the 3DS version being mainly from handheld games, while the trophies in the Wii U version will be from console games.
When it comes to music, each stage on the 3DS version will only have two music tracks available, as was the case in Melee. On the other hand, the Wii U version will see the return of Brawl’s My Music option, with a large selection of tracks available for each stage.
Sakurai has stated that there are currently no plans to implement downloadable content or touch screen controls of any variety. However, he did state that once the game is released DLC may be something they will take into consideration.(citation needed)
3DS version logo.
Wii U version logo.
Wii U Boxart.
A three-way battle.
New KO ownership trails.
An image showing the new visual indication of an especially powerful blow.
The "finishing move" of Fox's consecutive jab.
The first three confirmed newcomers
The difference between outlines off and on.
The new Team Battle outlines.
Official illustration of Villager.
Official illustration of Mega Man.
Official illustration of Wii Fit Trainer.
Official illustration of Rosalina.
Official illustration of Little Mac.
Official illustration of Greninja.
Official illustration of Palutena.
Official illustration of Pac-Man.
Official illustration of Robin and Lucina.
Splash art of Villager.
Splash art of Mega Man.
Splash art of Wii Fit Trainer.
Splash art of Rosalina & Luma.
Splash art of Little Mac.
Splash art of Greninja.
Splash art of Palutena.
Splash art of Pac-Man.
Splash art of Lucina.
Splash art of Robin.
Splash art of a Mii Gunner, based on Abraham Lincoln.
Splash art of a Mii Swordsman, based on Elijah Wood.
English splash art of a Mii Brawler, based on Ice-T.
Japanese splash art of a Mii Brawler, based on Shinya Arino.