You can load your 4K video into iMovie and easily export your QuickTime file in 1080p full HD. Here's how you convert 4K to 1080p on iPhone.
Step 1: Add a 4K video to iMovie on your iPhone.
Step 2: Click Done in the upper-left corner.
Step 3: Tap the Share button and click Save Video.
Step 4: Choose HD - 1080p as your output format.