I do not think Sonic or Snake will return for Smash 4. Here is why... lets us evaluate how Sakurai treats 3rd party characters. There were no 3rd party characters in his original entries, Smash 64, and Super Smash Brothers Melee. The first game to include 3rd party characters was Brawl. One at the behest of a friend, another due to sheer fan popularity. This time around we already have Mega Man. He correlates to Sonic. He was added due to staggering fan request. We do not know any other 3rd party characters that are going to make it. Most people assert that Sonic and Snake will make a come-back. Others think that Pac-Man has the connections to secure his imminent roster slot. This leaves us with 4 characters.
That isn't so bad. It seems reasonable currently. However, we must look beyond. Smash X is what I will refer to the Smash after Smash 4 and 5. This gets into some subjective territory as Sakurai might not be the creative director on the next project. So a vision change might occur. However, for the sake of discuss, let us assume that Sakurai is on board for Smash X. This is where problems start to occur. Sakurai likes to treat 3rd parties as a special case. Sonic and Snake were both special. Now Mega Man is considerably special. How special would Smash X be if he continued the pattern and followed precedent (for arguments sake assume we get Pac- Man and Mega Man for Smash 4)? This enables the Smash universe to have 6 third party characters.
Obviously 6 3rd party characters is borderline ridiculous when we considering that Sakurai already believes that the roster is feasible currently. At this point, the 3rd party characters will literally be taking up Nintendo roster slots. This isn't at all what Sakurai would want. In fact, I think Pac-Man falls under the "Snake" rule. We might get him in Smash 4 only because of Namco's association and involvement. So Pac-Man in Snake are more like favors or obligations that Sakurai has. Mega Man and Sonic are the 3rd party characters that the fans WANTED, WANT, and WILL WANT. So it is very possible that he might just cut off obligations and keep the 3rd party characters at two per entry (meaning 2 in Smash 4 probably Sonic and Mega Man) and keep the two during Smash X.