Many MP4 devices work the same way as MP players, so you can transfer files by a USB connection as long as you have the appropriate cord. Certain players might require a software installation to copy-paste music via a program, but standard file transfer is the rule most of the time.
Assuming your MP4 device is plug and play, you need to connect the player to a computer with the provided USB cable or another compatible one. USB 3.0 cables transfer data more quickly, so get one of those if possible. After connecting the player, wait a few moments for the computer to acknowledge it. You should see a popup box indicating that a new USB storage device is active.
Open the folder where your music is located and navigate to the desired files. Hold the Control key, and click each file you want to transfer, then right-click one of the selected files. If you want to remove the file from its original location, click "Cut."
If you want a backup copy, click "Copy." Navigate to the folder indicated by the MP4 player, and open the music subfolder if one exists. One likely does because Mp4 players play videos as well as music. Press Ctrl + V to paste the files in the directory. Eject the drive when you're finished.