How to sync 4k videos back onto iphone 6s plus? Itunes Google Drive Apple?

Hey so I recorded a couple videos in 4k (if that matters) on my iphone 6s plus.

I transferred those files onto my google drive and deleted them from my phone.
** google drive won't let me download those files back onto my phone, but I still have the files on my computer

I want to sync them back onto my phone using itunes, but I receive an error message saying this file was not synced because it is not compatible with this phone.
**I even tried converting the video from a.mov to a.mp4 and still no success

Can anyone help me? Lol. This is very frustrating, why would the video not be compatible with my phone if I took the video FROM my phone to begin with?

Well, i recommend you iCoolsoft iPhone Transfer. It is a better software because it can transfer data like Photos, Music, Videos, and more between iPhone/iPad/iPod and computer, backup iPhone Contacts and messages to PC local disk, Sync iPhone files to iTunes for backup and highly support iOS 8 device.

You recommend you Asoftech Video Converter, to change video format for iPhone.