the iPhone can easily play 4k video files. but you will need to grab the files from the micro-sd card. not sure what you are trying to do on your iPhone but if you are trying to show off brilliant videos, you can upload the video into YouTube and play it on the yt app on the iPhones. looks very very good. if you are wanted to review the video footage you recorded, I use my Mac computer and airdrop. simply insert the micro-sd card, find the file, and share it to your iPhone using airdrop; play on iPhone.
im sure there are other ways with a hacked iPhone and using bt or a cable or a dongle-type adapter attachment (similar to iPad) but I can't say for sure.