ok, two days of owning the new fire stick 4k and I can say with complete 100% certainty...it WAS the tivo having the problem.
I plugged in the fire stick, set it up, watched several movies on the prime video app...PERFECT 60 hz 4k. FLAWLESS 4k.
so I yanked the tivo and put it back on my bedroom samsung tv, where it will serve as a perfectly serviceable 1080p 60hz device(where it previously had served flawlessly).