i’m testing my last image (raspbian 2019-09-26) on Raspberry pi 4 with a 4k tv and it works fine. But the rasp doesn’t show anything after conversion with last version of mender-convert. I can use ssh to connect to it, so it works…
From the top of my head, I can not think of a reason why it would not work after mender-convert.
Which version of mender-convert are you using?
Additional I would compare the content of config.txt and /proc/cmdline before and after conversion, which might provide some insights if there are any differences.