The Media Player app on PS4 will be getting an update today, which adds 4K video support on PS4 Pro models. The update will allow 4K videos in mp4 format videos saved on USB or streamed from media servers to be played on the Media Player app on 4K TVs.
If you own the PlayStation VR headset, this update also means you can playback 4K VR content. PS VR’s display resolution is 1920 x RGB x 1080, therefore 4K VR videos will be displayed in a higher image quality compared to HD VR videos. Capture stunning 360 degree 4K footage, and then immerse yourself in that footage in VR at home – just as if you’re right back in the scene you shot.
External USB storage that’s already been formatted as extended storage for your PS4 (using Firmware v4.50) can only be used to save games and apps – it can’t be used to save and play the videos.