While still a fun game, it comes off as an extremely lazy sequel.
Story:
It's like the old Sonic games, you really think there's a story? Basically Eggman wants to destroy Sonic and felt the best way to that was to reuse previous machines. Well Sonic just won't let that happen.
Gameplay:
The game play is 2D Sonic, however it feels more like Sonic Rush. What I mean is the physics in this game are ripped almost directly from those two games. You can scale walls with ease, stop on loops, rings bounce further the more you get hit, and speed is INCREDIBLY easy to obtain. That said though, the gameplay works and is still quite a blast. The most significant change from the classics is the addition of the homing attack. It works well and I really don't find it to be much of a problem. However, when you clear all the special stages, you can use Super Sonic in the stages. I HIGHLY recommend doing that as the game becomes instantly more fun.
Graphics:
Surprisingly enough, the game is quite pretty. It's colorful and lively, exactly what you expect from a Sonic game. The level design is nice, but it's all reused. We have NOT Green Hill, NOT Casino Night, NOT Labyrinth, and NOT Metropolis. Yeah, lazy. Still though, they look quite nice.
Sound:
The sound effects are typical Sonic fair. The music is okay, but nothing special. The Super Sonic music I love though, but the final boss music is disgusting.
Overall:
Sonic the Hedgehog 4 is still a fun game despite my nitpicks. Though honestly it doesn't feel like "Sonic 4." The games selling point is Nostalgia here, but since it feels like Sonic Rush that kinda fails. While an extremely lazy sequel, Sonic fans should still have fun with this game.