How to convert MP4 to MKV? Go to 'FileConverter.digital'. Select a file in step 1. On step 2, select 'MKV'. On step 3, click 'Start' to begin converting your video. 4. Provide an easy interface to convert MP4 to MKV files within 3 steps. Step 1. Go to FVC Free Online Video Converter, you can click the Add Files to Convert button to add the MP4 file into the program. If it is the first time you use the online MP4 to MKV converter, you should download the installer beforehand. Step 2. myPoo.