According to the wiki I should be able to use --multitap to connect a multitap for 4 player games on SNES cores, but this just gives a parse_input error at least on the hunterk ppa build. The core options don’t expose the choice for using a multitap and it doesn’t look like the 3rd and 4th controllers get detected automatically so is this actually possible?
Also, along the same lines, is it possible to use gensgx plus to play j-cart 4 player games. The only options it exposes in core options are 4-wayplay and teamplayer.
Edit: In fact, more than 2 player support seems a bit spotty across the board, I couldn’t get 4 player working on mednafen psx either. Looks like unless the platform had native support for 4 players it doesn’t work. Conversely on Mupen64 I don’t seem to be able to have less than 4 players regardless of how many controls I actually have connected (at least on StarFox64). MAME seems to be the only reliable one out of those I tested.