AVI does not identify the codec inside
AVI is a container that can hold video and audio encoded in different ways. Older files may use codecs that modern phones, browsers, or editing applications do not include. Identify the source streams when possible and confirm which MP4 codecs the receiving device supports.
A conversion may decode and re-encode the content rather than simply changing the container. That distinction affects quality, file size, and processing time. Keep the original AVI until the MP4 plays in the target environment and the needed tracks are confirmed.