you need 2 16x lanes for quad SLI. All Intel chipsets are x8 x8, no x16 x16. Thus the only motherboards that support it are the nVidia chipsets (which arent that great for Intel to begin with) to ATi's RD600 (which seems like a beast). Its your pick of the poison. It wont be long until ATi releases a Quad SLI counterpart, im pretty sure.
EDIT: And as much as i love nVidia and ATi, i think Quad SLI is the biggest waste of money. I would much rather go with 2 X1900XTXs, better yet, 2 X19050XTX when they are released. Can't wait for R600 and G80
EDIT2: Thanks Jamxx