I was wondering how to get 4k video,when I download it from my iPhone 7plus it just says 1080p as a option .
I have the DJI mini 2

You'll probably receive better answers if you ask in the DJI Mini 2 forum.
Are you sure the iPhone 7 can handle 4K video? Could the resolution be limited because of the limits of the phone?

Hello there Gord1. I am sorry for the trouble this has caused and thank you for reaching out. Can you please check if the said mobile device ( iPhone 7 Plus ) that you are using can process and upload a 4K video? In addition kindly please try using a different recommended mobile device for the DJI Mini 2 for us to be able to isolate the said issue? Thank you.

https://www.dji.com/mini-2/downloads