I think it depends much on whether or not you enjoy doing any or all the hardwork related to making SLI works. There are people who actually do not see hacking drivers, managing heat..etc as hassles but fun times. If you do enjoy the process as much as the result, maybe you should.
You could achieve close to what you have described (4K highest settings at 60Hz in most games) with a single Titan
V.
Reference:-
https://www.hardocp.com/article/2018/03/20/nvidia_titan_v_video_card_gaming_review/1
Unless your last name is Gates or have won a lottery, we mortals usually cannot go down this route.
As said above, I think the 1180 Ti might give performance close to Titan V but 'relatively (only)' cheaper. 1170/1180 might come out, say, in August and then the 1180 Ti might come out 9-10 months later. So, optimistically speaking, this 1180Ti will come out around June, 2019.