Convert HD video to 4k with iMovie

I recorded daughter’s musical with regular HD @30 FPS during act 1 on accident. There were about 4 files because I would turn off the camera during black outs. For Act 2 I realized mistake and recorded 4k @60 FPS. Got home, added all files to iMovie then exported, which took several hours. Is that normal? Runtime is 1.5 hours. Anyway, the result is HD @30 FPS throughout. I saw thread where if I add one of the 4k @60 FPS first do something in settings, after adding the other files I could remove the first high quality file and the movie will somehow be 4k @60FPS. I can’t seem to figure out how, and the exporting process literally takes more than 5 hours. If I’m am mistaken and this can’t happen, could I bypass the need for iMovie and just take one 4k @60FPS before sending it to YouTube, or will my upload take a hit on resolution or FPS because of the large file size? Would it be best to record several high quality videos and just upload a playlist to YouTube?

Thanks everyone