Hello!
First of all, this problem happens only when you export in 4K? What happens if you try to export in another format? (Because maybe is not the resolution but the camera what it does the problem.
Check that the “Camera Cuts” track is in the sequencer and that the camera that you want to render is in the Timeline. That is what Unreal will be render.
If any of these are not:
Track → “Camera Cut Track” will give you the “Camera Cuts”
And in Camera Cuts click the “+ Camera” button and add the camera that you want.
I hope it helps you!