At some point you guys stopped reading... The OP will not put 12 GPUs. He only wants 4
RM850x for example is enough. RM1000x is better and more expensive.

Now ... about the rest of the parts.

SSD 120GB minimum, 240GB is better and price difference is minimal
RAM 4GB is enough, 8GB is better and the price difference is minimal
MOBO do you plan adding more cards? If not, just buy a 4-5 slots one
CPU please please please do not buy Celeron. For $20-30 you can get a Pentium

Out of all this stuff, the PSU will be the most expensive part.