So a megadrive/genesis title then.
The 4 players thing is achieved with a multitap and unlike some I think it was racing games I am not aware of anything in the cartridge itself.
Video for others, there are off screen characters/sprites to handle.https://www.youtube.com/watch?v=4v3WVmXbUkY
To that end you are in for a serious hack if you want to do this, and may will have to delve into electronics or coding an emulator specifically to handle it. I don't see a reason why you could not kick control of a sprite to another portion of memory which a controller is read into, and thus have your mod, but the stumbling block will be figuring out how to get the controls in. What might work is find a lua supporting emulator (not sure what the kids are using on the megadrive for this one but tool assisted speedrun forums are the usual place to start looking here) and then get it to manually inject results of keyboard/joypad/whatever presses into some free space in the game's memory and then altering the AI routines (or indeed removing them) and replacing them with the key presses.
Had you just wanted to do 1+2+3 vs 4+cpu1+cpu2 then we might have had something to discuss, indeed for the above hack I would probably start by doing that as figuring out how to do that would get you 90% of the way there other than the getting extra controller inputs to be recognised.