If the MP4 video is not encoded with PS4 compatible codec, we will fail to stream MP4 to PS4 or cannot play MP4 on PS4 even if successfully transferred MP4 to PS4. You can change MP4 into playable one and follow the steps below to copy and play movies on PS4:

Insert USB Drive into USB port, turn PS4 on and then go to the main menu, select “Video” and find your USB Drive, select “Display All” to see the video files that exists on your PS4, please choose the file that you want to transfer and press the triangle button, choose “Copy” and confirm the transferring, the movies are transferred to your PS4 hard drive, and then you can play MP4 movies on PS4 from USB Drive.

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