@ben,
you mean 4 players in split screen then playing 8 other players via wifi....
I don't think so. From what it sounds like you have:
play solo on your wii vs cpu.
play solo on your wii vs 1-12 players wifi
play 1-4 on your wii.
This is ok, though the prospect of playing a few players in the room and a bunch of others via wifi would be really nice.