Final Cut: Youtube 4K Upload directly or by file
We cannot tell by the information you're giving. The "upload to youtube" function uses some sort of container and some sort of codec with some sort of bitrate that we don't know, however, 8Gb for an hour in 4K is a very small file. I remember exporting a shortfilm at about 24 Minutes in 1080p and having a 15Gb file.

Youtube converts your file after you upload it, so rendering in a semi-okay codec will force youtube to make an even worse file out of your rendering. If you want to be absolutely sure, just render your file as a ProRes 4444 XQ and derive all conversions from there.