I have a motherboard that has five PCI slots (two PCI's are the small version). It didn't like four 750 Ti's, but was fine with three 750 Ti's and 960. Do you think this is a Windows thing out of curiosity?
Did you plug in all cards in at the same time ?
When using windows i always start installing the cards in the pci-e port nearest the cpu. And then adding more cards one by one as long as they get detected by windows. Sometimes it can be a real hazzle before windows find all cards, i even have to move cards around on the pci-e ports to get all cards going. So, plugging all four cards in there and installing the graphics driver will probably not work right away.
Make sure you use a powered riser in the x1 slot if the 750ti doesn't have an external power connector.