So the Xbox 360 version of Sonic 4 has been leaked already, and I have had a chance to play it and it is rather good. There is no story explanation, voice overs, or anything like that, just classic 2d Sonic. The game really feels a lot like a high-definition Sonic 2, especially the casino.
The music is somewhat old school, especially the main theme, with a lot of synth. There are 3 acts per zone, and 4 zones. There are of course 7 chaos emeralds, which finally let you play as super sonic in the levels. You can only earn one emerald per level (by having 50+ rings at the end), but you can play individual levels as many times as you like. The special stages are like the ones in Sonic 1, only much more difficult since you always need to collect a certain number of rings.
Overall, it is a solid game that finally brings Sonic back to the good old days.