Considering the WiiU/3DS's power, and considering how many characters are sure-fire to return as well as how many of the series contained in Brawl that were most likely maxed out on spots WITHOUT realizing increased spots for WiiU/3DS (Mario, Zelda, Pokemon) or were just starting to include new characters at all (Kirby, DK),
It seems to me about 24-26 of Brawl's roster will be back, and the rest will be newcomers. Brawl has 35 characters; I can't really imagine how much of an increase in spots we'll have.
As a pointless comparison to other crossover games, UMVC3 has 48, though it's to be noted that this was after having a rerelease. Without inclusion, it leaves at 36.
Street Fighter x Tekken includes 60, but only after including each and every DLC character playable. Without inclusion, it leaves at 38.
Sony's Playstation All Stars Battle Royale also includes 48, but only after releasing DLC. Without the two, it has 46.
Can these three results be of any lead towards what we can expect? I believe they might, as PASBR includes another 11 characters than Brawl did and was the latest released, while the previous crossover games all had 34-38, including Brawl, which had the least playable out of each game even without the consideration of the other games' DLC. This leaves me to expect at least 10-15 more characters than what Brawl had to offer, realizing that PASBR is a game that is very close to Smash Bros.
This would leave us at 34-41 characters, the lowest being the least likely as it's less than what Brawl gave us, and the highest being DOUBLE the amount of "sure-fire" characters to return to us in Smash 4 I could come up with, off by just 1 slot.
Of course, 41 is being hopeful. I would be fine with simply 35, same as what we had when Brawl was released. But we do have to remember that, yes, if the 3DS can't handle the amount of characters that the WiiU can (for whatever reason, I have no knowledge on this kind of thing mind you), then we would be limited.
(Made a quick edit, typo near the first sentence of the final paragraph.)