The whole process is divided into two parts. First mount IMG file just as if there was a physical DVD disc in your drive；then convert the DVD disc to MP4.
Mount IMG files
Windows 10/8.1/8 have built-in Disc Image Tool that supports mounting IMG file as a virtual drive. Just mount IMG files as follows:
Step 1. Open the IMG file folder.
Step 2. Select IMG file and it will show the Disc Image Tools Ribbon, next click on Manage > Mount. Or right click IMG file and select Mount from the context menu.
For Windows 7/XP, you need install a third-party image mounting software like DAEMON Tools Lite. After installation, run the image mounting software and follow the instruction to mount your IMG file, which is quite simple and here are 5 free options of virtual drive.
Once you mount the IMG file successfully, you can go to Computer and find the virtual DVD Drive.