I disagree with the premise, because it assumes the possibility that Sonic 4 is good. Sonic 4 is simply a bad
game no matter how you spin it.
First and foremost, the way it controls is unacceptable for any platforming game,
period. Forget about bad rolling physics; everything including simple running and jumping feels heavy and awkward. Its almost like a chinese bootleg created by people who have never touched a controller. Im told Sonic 4 uses the same physics as Rush, but the basic movement doesn't feel as bad in that game. Im not really sure why.
Level design is focused around stage gimmicks, yet never really escalates them the way something like a Mario game would. When its not blandly using gimmicks, its rehashing Rush-esque level design without the mechanics and pacing that makes that kind of game fun. This leaves behind shallow stages that wear out their welcome long before they're done. Don't forget about the homing bridge over a pit crutch that just about every stage ends with!
Graphically and musically Sonic 4 is a putrid mess. The visual style is wildly inconsistent, something that is evident in any screenshot of the game. Sonic, the world, the HUD, the enemies - they all look like they were pasted in from different sources. Sources that incidentally, all happen to be resoundingly awful. The characters have an absolutely ugly cel shading applied to them, and the world looks like someone took the prerendered blandness of NSMB DS and cranked it up to eleven. The music isn't much better. A chunk of the soundtrack is utter shit composition-wise. Granted, there are some good compositions but these are betrayed by everything sounding like dying cats.
At the time of release, Sonic 4 was fifteen dollars on console platforms. This is more than the price of a movie ticket, yet the game offered about 45 minutes of mostly mediocre gameplay sans potential replayability (which personally was low due to game being terrible).
There is nothing about the package that is in anyway fully acceptable. Its a bad classic Sonic game, its a bad Sonic game, its just full on a bad game.