I could see a 4-Player Mode happening, but only with something like Co-op Versus...Basically like a 2 vs. 2 type thing. The top image would be a good image for that, then.
A 3- or 4-Player Versus mode might be possible, but I don't really see how...unless there's something like last one rocking or whoever has the most points/highest accuracy etc... For this the bottom two images look better. Maybe side-by-side fretboards will look good, too.
I'm very used to Marioparty style mini-games, so personally, I think that in 1-player, you have the standard screen; 2-player, you have to vertical split-screens; 3-player, either side-by-side vertical screens or each player having a quarter-size standard screen with the last screen being settings or a "live" screen; 4-player, either side-by-side vertical screens or each player having a quarter-size standard screen.
This is just my personal preference, but maybe one of the other images may look good in-game.