There are motherboards that can support 4 cards. For example, Gigabyte has motherboard GA-890FXA-UD7 with extended form factor that supports four double spaced cards: http://www.tomshardware.com/reviews/amd-890fx-crosshair-iv-formula-890fxa-ud7,2640-6.html.

However it is required to have the special case for such motherboard (XL-ATX).