Well, there are quad Xeon motherboards. SuperMicro and Tyan specialize in that. They're quite large though--most won't fit in just any ATX case. Often you end up needing something like a
SuperMicro SC850.
Traditionally for the really large x86 boxen (4-8 way with >16GB DIMM capacity), the motherboard is separated into multiple boards with proprietary mount points and interconnects. Closest I've seen to that is an AMI 14u quad Xeon barebones rackmount (I can't seem to identify the model or anything, sorry). The case itself was solid steel with several hefty hot-swap power supplies, twelve hot-swap half-height SCSI bays, and a three-piece motherboard design. The main part of the motherboard held four Slot2 mounts for P3 Xeons plus 16 PC100 slots and was mounted vertically. (It appeared to have a SCSI controller onboard as well--perhaps an AMI MegaRAID). It had one daughtercard plugging into a proprietary slot and mounted horizontally with 16 more DIMM slots, and the top edge plugged into the backside of an active backplane with some 10-15 PCI slots.
It was a sight to behold. It was also gloriously inefficient in the amount of rackspace it consumed. 14u!!! God damn. Of course, the enormous Slot2 Xeons probably had a lot to do with that, but I think AMI could have arranged to pack a lot more in there.