Hi, AppleMaxi,
You probably should ask You Tube what is causing the problem.
iMovie does not support project export at 4k 60ps. iMovie does support project export at 4k 30fps and 1080p at 60fps. iMovie will reduce clips that are 4k 60 fps to 4k30 fps in the timeline.
I'm not sure, but I believe that You Tube does support 4k. Check the You Tube website for the supported video specs. I am guessing that iMovie reduced the movie to 1080 60 fps on export since it does not support 4k 60fps.
Instead of using iMovie's direct upload to You Tube feature, try exporting your project to your desktop at 4K (it will be 4k 30 fps), or export at 1080p 60 fps if the frame rate is your priority. You will have those resolution options in the iMovie export box. Then drag the movie into You Tube's upload window. See if that works.
-- Rich