I am trying to render a 4k sequence and regardless of which level of quality I choose it renders a completely black video sequence. 1080 rendered at any quality works fine. These are rendered from the same intermediate files from Lightroom. Any ideas? I'm stumped.

Which player, which system?
Most players have problem with 4K. It's most likely not the video file.
If you are on Windows, use mpc player: https://mpc-hc.org/
on mac try VLC.

The 4k files don't work in quicktime - but the lrt3 4k renders always did? They do work in premiere though so who knows