You don't need 8x risers, just use 1x.
750W is enough for 4?. You are missing relevant information, which graphics cards? Probably not enough, unless you use cheap crappy GPUs.
1x riser fit in 8x slot? Have 5770 6750 6850 6950 gpus
Yes, just fine.
Look up the TDP of those GPU on google, then add them together. Add 125W for the motherboard/cpu and you have a PSU estimate.
That many different GPUs in one computer is going to suck, bad! Very hard to configure properly.