The story is Dr. Eggman the main villain has been collecting animals and turning them to robots, and sonic must stop him doing this at all costs. The story in Sonic 4, continues where Sonic & Knuckles left off. In sonic 4 Dr. Eggman returns to get Sonic once and for all using all his know how to build some powerful and destructive machines.
Expect to many returning enemies from the earlier games. Motobug the ladybird type robot on a wheel. Obinaught the ball surrounded by 4 moving spikey balls. Chopper the robotic fish from the Greenhill zone that jumps and bites you when crossing bridges.
Some of the confirmed levels are Splash Hill and Lost Labrynth.