Step 1. Download Handbrake. You can get it from the official website here: Handbrake.
Step 2. Click "Source" button and select your M4V videos.
Step 3. Choose the folder where you want to save your outcome movie.
Step 4. In the output settings, choose MP4. You are also allowed to choose the frame rate, video codec, bitrate.
Step 5. Press "Start" to convert M4V to MP4.