sonic "4" AKA sonic crackers does exist, as a beta (actually more of an alpha). it's basically a Chaotix-style game. never completed, there's only a level or two playable (which is still missing most parts too).
The rom that u see around of sonic 4 is pretty buggy .. I get stuck in air sometimes and walk on nothing .. its obviously incomplete .. but deffinately a real game by sega. And btw you said you were sick of playing sonic1-3. Why dont you try Sonic CD, Sonic & Knuckles, Sonic Spinball ?