Both Metal Sonic and Amy Rose were introduced to the game series in Sonic CD. Could that be the inspiration for Sonic 4? Sonic CD was originally developed as a port of the original Mega Drive game ...

Sonic 3 features a lot of characters. We're talking major parts for Sonic, Tails, Knuckles, Tom, Maddie, and Agent Stone; the introduction of Keanu Reeves' Shadow and Krysten Ritter's Director ...