I've 3 small rigs with 4x Geforce 750 Ti. Some infos you might find useful:
- use powered risers.
- it's a bit tricky to fit all of them in. The bigger the case, the easier. Do it the MacGyver way and improvise.
- the GPUs mentioned above are the ONLY ones which will do, everything else will overheat. You may still need an additional fan or two.
- try to find 750 Ti which have a dedicated power connector (most do NOT have one). This reduces the load for the motherboard.