4K iMovie won't render in 4K on Youtube

[Scion142]

Now that I have a new iPhone 8 that can shoot 4K video, I tried it out and imported the footage into iMovie. I experimented with 4K video clips only for my first "project" and it uploaded to Youtube and it rendered in 4K (actually 1440P) which is fine. For my next project, I mixed some 4K video clips in iMovie with some photos I shot at the same time. Following instructions, the first clip in the project was a 4K clip and then 4K clips and photos followed. When I hit "settings" in iMovie it indicated the project was in 4K. But after uploading it to Youtube, it was now a 1080p movie. What happened? Did mixing photos shot with the iPhone 8 with 4K video clips cause Youtube to default to a lower resolution? The photos are shot at higher resolutions than the video so I can't figure out what is going on! Any help would be appreciated!!

 

[DeltaMac]

I was viewing a youtube video that said you can upload 4K, but not view it at 4K until after a delay. That video said it might take as long as a week for the 4K to be available for viewing.