If it didn't create a financial burden, then I can't say it was a stupid decision. On the other hand, its hard to see how owning a 5D3 and a 5D4 is practical in your situation. Even a 5D3 and 7D2 would be more practical because of the 1.6 crop factor, if nothing else. With that said, everybody has made impractical purchases because they simply want something regardless of how much sense it makes to have it. Did you buy the 5D4 because it you wanted the latest and greatest, or did you buy it for some other reason?
Reason for having a 5d3 and 5d4 is for the occasional wedding gig. I'd rather have another full frame as backup.
My main reason for buying the 5D4 was for the extra resolution, dynamic range, DPAF which now allows me to take some video with my DSLR...something I was never comfortable doing before since manually focusing video is quite difficult.