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?
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.
this is all good advice for troubleshooting windows based issues ...
if you try your hand at linux - let me know ... it has almost no issues recognizing cards - but the drawback is you dont have the flexibility for overclocking that you do with windows ...
but then again - linux is free too ... no matter how many machines or how many times you install ...
...i use fedora 22 x64 with cuda 7.5 ... compiling ccminer-spmod is a breeze in linux and is reporting the same ( and sometimes better ) hashrates than cuda 6.5 ...
if windows is still your base - then a lot of veterans here can help ( and have already answered this question ) ...
#crysx