The fact that Black and White were released after Brawl really do not hold much, if any, importance. Especially since the Ruby and Sapphire remakes come out around a month before Smash 4, based on the estimates that Nintendo has given.
As I mentioned earlier, we already have both a fully evolved fire starter, and a fully evolved water starter. Therefore, it is not illogical to assume that we shall see a fully evolved grass-type starter.
Since we have that established, we have to take a look at the fully evolved grass-type starters throughout the Pokemon games. Many of them are large quadrupeds (Venasaur, Meganium, and Torterra), which probably are not suited for a fighting game. Serperior is unlikely because of its snake-like body structure, and the difficulty of creating moves that comes with such a structure. This narrows it down to Sceptile and Chesnaught. I would not be surprised if Chesnaught were included in Smash 4. However, I feel that Greninja's inclusion in the game hurts Chesnaught's chance of making the cut, because they are from the same games. This pretty much means that Sceptile is the most likely to appear as a playable character by default.