Sega may have you thinking Sonic 4 is on the way, but really, it's already here. It was released in 1996, for the Super Nintendo.
This is a game called "Sonic 4", a hack which took the existing Speedy Gonzales: Los Gatos Bandidos (by Sunsoft) and added some extras to it. Like Sonic, Sonic's rings, and little Marios in cages, which Sonic has to rescue.
Sounds bonkers, I know, but the proof's in the video.