You can't have 4 cards in SLi. Max available is 3 on a 780i board, or 2 dual-cards ( as in, 2 chips on the same PCB)
Orly? I count 4 PCI-E slots on this board: ^MSI X48 PLATINUM^ MSI P7N Diamond also has 4 pci-e card slots
you can on a 38/x48 board if they include a bridge chip(or you manage to get your hands on un-f'ed up drivers)
Does not matter how many PCI-E slots a motherboard has, unless it has SLi support (which only nVidia chipsets do) then it cannot run SLi. Theoretically, you could stick half a dozen PCI-E slots on a motherboard, but unless you planned on running 12 screens on your PC, it would make no difference.