Hey lassi290,
We'd recommend reviewing the information in this article as it provides information on recording 4k video: Record HD or 4K video with your iPhone or iPad
Depending on your iPhone or iPad model, you can record video in high-quality formats, like HD, 4K, HD (PAL), and 4K (PAL).
To change the video format and frame rate:
1. Go to Settings.
2. Tap Camera, then tap Record Video.
3. Select from the list of video formats and frame rates that your iPhone or iPad supports.
On iPhone XS, iPhone XR, and later, you can also tap the quick toggles in the top-right corner of your screen to change the video format and frame rates.
PAL is a television video format used in many countries and regions in Europe, Africa, Asia, and South America. PAL formats are available with iOS 14.2.
When you open the Camera and select Video, the format that you chose (for example, "4K • 60") appears at the top of the screen.
Also, take a look at this page, as magnetic accessories can cause camera issues. If you have a case, remove it and test, just to be certain.
Magnetic accessories might interfere with iPhone cameras
If still not working as expected, we'd recommend contacting Apple Support here: Get Support
Thanks!