can't add mp4 to itunes

See iTunes: Frequently asked questions about viewing and syncing videos - Apple Support. iTunes will play MP4 video files if they are compatible with / playable in QuickTime - which many, but not all, MP4 files are. MP4 files may contain video data encoding using codecs that are not QuickTime-compatible - these are not playable in QuickTime.

You should, though, make sure that you have the current version of QuickTime (7.76) installed, and it may be worth verifying that it is installed correctly by selecting, in Windows, Start > Control Panel > Programs and Features, finding the entry for QuickTime, right-clicking on this and selecting Repair. If none of these have any effect then your MP4 files are probably not iTunes-compatible.

Unless you can go back to the original source and re-encode the video using a QuickTime-compatible codec, you'll have to find some method other than iTunes to play them.

Unless I'm misunderstanding something, if they play in iTunes then they are in the iTunes library. In iTunes 12 (most) video content is added with Home Videos as the media type - have you checked in that category? Select Movies (top right of the screen, using the ellipsis (...) icon if necessary to see it) - there should then be a Home Videos entry in the list across the top of the window.

That being said, not everything that will play in iTunes/QuickTime will necessarily play via Apple TV. You might want to repost your question to the Apple TV sectionof this site if that's the actual problem.