o hey! funny you say that! i had the SAME situation, here's how it went:
x4 1080ti will NOT even BOOT windows 10 , it just kills the PSU, powers off. (1000 watt G2 GOLD), i HAD to move to a bigger PSU (i put on a 1600 watt Hercules)
i was able to ADD another 1080ti once i had the 1600 watt PSU, and after tuning, it only pulls 995 watts for all 5 now
I assume it needs to pull some full draw on boot maybe?
Please let me know if you get 1000 watt PSU to boot x4 1080ti (one of mine was a zotac they are 320 TDP, more than a usual 1080ti)