Doing video edition with clips recorded at 4K 60FPS, these are viewed in high quality on the Premiere Pro editing screen, but when exporting using the same sequence settings, the same speed and H.264 format, the result is that dark backgrounds become very pixelated, and the quality of the final video is terrible. I've tried different export settings without success.

Maybe a question for Adobe?

Hello there. I am sorry for the trouble this has caused and thank you for reaching out. Are you able to post some images showing the said issue and I hope that our fellow DJI co pilots who uses the Premiere Pro in editing there work can give out the best recommendation for this issue. Thank you.

I'm not familiar with the settings in Premier Pro but in general you have to remember you are not copying the clips but editing them so they will be compressed again when exported. To avoid quality loss it is often quite normal to set the export settings to higher bit rates to help  prevent too much compression. If Premier has it you should also set the quality settings to 'Best'. The overall bit rate for the file should be at least 91.5Mb/s  for H264  at 4K in my experience. Going over than does not seem to add any significant increase in quality.