My main complaint about Sonic 4 is that it mostly copypastas things from the first 3 games (especially Sonic 2). It feels uninspired, like they looked at the best bits from the older games and just copied them directly, instead of looking for inspiration from the old games but creating something new. I completely understand Sonic Team playing it safe, (even though the Sonic "fandom" is probably the single worst fandom in existence and there is absolutely no way they can be pleased) but they could at least toss in some new environments, instead of just rehashing Green Hill/Casino Night/Sandopolis/Labyrinth/Metropolis/Death Egg.
Also, the music is the most uninspiring and dull soundtrack I've heard in a Sonic game. Very disappointed in that, considering Jun Senoue is one of my favorite composers.
Stevo, I will agree that Sonic 4 is a step in the right direction, but so far, it's only a very small step.