There was a 2010 2D platformer released as Sonic 4 which was meant to be the spiritual successor.
I’d say the real spiritual successor on Genesis/Megadrive was Sonic & Knuckles, which came out after Sonic 3 and for all intents and purposes may as well have been called Sonic 4. But they had to push the Knuckles aspect because the cartridge had a passthrough that would accept another Genesis cartridge and allow you to play e.g. Sonic 2 with the Knuckles sprite, iirc.
It’s not though? Sonic & Knuckles has unique stages and story vs. Sonic 3. Unless you mean they were designed as one game and split at the end before release; that I don’t know.