I've seen dedicated mining machines with 4 video cards installed, but I can't seem to find cheap mobo with more than 2 PCI-E slots. Do you have to purchase a high end motherboard to run over 2 cards on the same pc or am i missing something?
Actually you can't install more than 3 cards in ATX case because it allows only 7 expansion slots and each card needs 2.
Moreover, you can't place them next to each other because one card will block the air intake or anoter.
You are looking not for "PCIe" slots, but for x16 PCIe slots, that's not needed for mining. People with 4 cards in one MB usually connect them with flexible x1 PCIe extenders. Some of them even work. That's better for cooling too.
Also you can't power 4 good cards with one normal PSU. You'll need an expensive one or two good ones.
And Windows doesn't supports more than 4 GPUs. 4x5970 is like 8 GPUs, too much :)