Hi, swaprin.
Thank you for visiting Apple Support Communities.
I understand that you are unable to add MPEG-4 files to your iPad Pro. I would need a little clarification on how you are importing the file to iTunes to possibly provide a better answer. However, let's make sure that you are importing the file to iTunes via the steps below. Once done please attempt a manual sync of this media instead of a drag and drop.
Import music and video from a computer into iTunes
iTunes 12 for Mac: Sync iPod, iPhone, or iPad
The next steps would be to verify if the media plays in iTunes and creating a iPad version of this file outlined in the steps below. Once done attempt another manual sync again.
- Open iTunes.
- Select the video that you want to convert.
- Mac: From the menu bar at the top of your computer screen, choose File > Create New Version > "Create iPad or Apple TV Version."
Windows: Hold down the Control and B keys on your keyboard to open the iTunes menu bar, then choose File > Create New Version > "Create iPad or Apple TV Version." Learn more about menus in iTunes for Windows.
If your videos from iTunes can‘t be played on your iPhone, iPad, iPod, or Apple TV
Cheers