It is enough Corsair HX1200 Platinum psu for 4 1080 ti?
It depends on the algo you mine and overclocking settings, for example equihash, ethhash has stable power usage but skunkhash, timetravel algos have enough unbalanced load, so 1080 ti can consume up to 300W, so HX1200 is next to one, but for downvolted cards it could be enough
Totally fine. My rigs are 5x 1080ti on Evga 1200P2.