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. It was available as an illegal bootleg in some of the world's shadier marketplaces.
Sounds bonkers, I know, but the proof's in the video.