Oop, my bad!
Yes, I have played Gens games with four players online before and it works.
I've played Mega Bomberman with four players (I suck at all the bomberman games, period, though they are fun. Likewise I've played that four player war game by Electronic Arts (the name escapes me at the moment, but it was that cartoony game with the red & blue infantry dudes fighting each other).
I've seen the 8 Player Bomberman Demo, but it seems only four of the 8 are controllable (at least messing around with gens offline). I've never tried it online before. As far as I know that's the only Genesis game that's more than four players. At least that I've ever heard of...
To revise my statement about Mame, there is one working 6 player game (Hard Dunk & its clone), and one 8 player game (Sprint 8 and its clone "Sprint 8 tag & chase"). There are 215 (not including clones) 4 player games (181 listed as working). 51 three player games (not including clones) with 44 of those listed as "working." And of course 1,822 2 player games listed as "working" (not including clones). A tiny fraction of these are one on one fighters. ; )
The Sega Genesis didn't have too many four player games (most consoles didn't and still don't), unfortunately. There was EA's "Four Way Play" that was only compatible with EA games, and the "Team Player" put out by Sega that only worked with Sega Games. Finally an adapter was released that worked with both types of games, but that caused some confusion there. Also there was a series of games called "J-Carts" that had extra control ports on the cartridge itself (I don't know if this has been emulated yet by any genesis emulator) so you didn't need to buy any adaptors to play with four players.
I don't know all the J-Cart titles (they were all sports games though), but here's the other ones I remember:
That military game (sorry forget the title)
some tennis game
NBA Jam (of course!)
NBA Jam for the Sega-CD (slightly different stats and tweaks)
NBA Jam: TE for the genesis
NBA Jam: TE for the 32X (better graphics and some tweaks and new stats)
There may be another couple of games I'm forgetting (most likely sports titles), but those are the ones I remember, and of course that 8 player bomberman demo, but that wasn't publically released that I know of, we're just lucky somebody dumped the cart for emulation. ; )
As for how it would have worked, I'm sure the original cartridge accepted input from two Sega Team Players (or EA Four Way Player's) connected at once, since each one used only one control port to plug in.
The Adapter that fit both has two ports and takes up both ports, but theoretically you could just unplug one of them and use two of the special adapters (sorry, I forget if sega released the new adaptor or not, I own it, but it's in a different city from where I am now).
PS: Never heard of Yuyu Hakusho! (hope it doesn't require knowledge of Japanese), but I'll check it out, thanks!
Set your mojob mame color in display to match your stupid ass desktop color to avoid black screen in netplay, fukaz. t(';'t -TFR, AD 2-20-02