Castlevania 3: Dracula's Curse. Every character is playable at the same time like in Bloodstained: Curse of the Moon.
The easy way to get this is play in an emulator like FCEUX and freeze address $54E with the number you want.
00 - Trevor
01 - Sypha
02 - Grant
03 - Alucard
Gamehacking.org told me this.
If you want the four characters to be selectable in the game easily, I'd have to find out where in RAM the currently-available characters are kept, which would mean playing through the game. No thanks.
Alternatively, use Game Genie code KVAVPY
to enable a debug menu: the first digit chooses the level, the last digit chooses who Trevor partner's with. Again, thanks to gamehacking.org for that.