i think you really only need 2 pci-e slots on the motherboard, a mATX motherboard can only fit 2 double slot GPUs. why are you looking for a small mobo with 4 pci-e slots? unless i missed something 
Use PCI-E Ribbon cables and custom built case. Cards can then hover far above the motherboard and have more than enough space between each card for ventilation. The only problem that may arise is each PCI-E slot is specced to supply 75W of power, and fully loading all slots may cause serious problems. You can get a PCI-E ribbon cable with a 12v molex connector wired into the ribbon cable to supplement the power to the motherboard, but they aren't exactly cheap.
