As I understand it (those are weasel words so you can't blame me if I'm wrong) some mpeg4 formats (e.g., Xvid, frequently found in an AVI container) have an aspect ratio specifier in the file and this may need to be changed to 4:3. Try mpeg4modifier, a little (30kB) free program that can do things like change the aspect ratio. It will be quick -- you are not re-encoding the file. I've experienced this encoding to AVI/XVID using AutoMKV but I didn't use mpeg4modifier to fix it. I just re-encoded using the feature in AutoMKV that allows you to specify the Xvid AR you want. Another handy small free program is mediaInfo. With it installed you can right click on a video and it will tell you the properties, including the AR in an Xvid video.