What AVI to MP4 actually writes
Phones, WhatsApp, and websites reject AVI or play it with no sound.
The AVI is re-encoded to MPEG-4 video and AAC audio in an MP4, with the header placed for streaming. Subtitles and odd extra tracks are not kept.
Quality and the AVI file you should keep
If FFmpeg cannot read the codec inside the AVI, the conversion fails. A new MP4 cannot repair a damaged or tiny source.
A related step, if you still need one, is MP4 to AVI.
When AVI to MP4 is the right page
AVI to MP4 sits on this site because AVI and MP4 are not interchangeable, even when they look similar in a file list. Phones, WhatsApp, and websites reject AVI or play it with no sound. Old lab software and some Windows-only tools still ask for AVI and reject MP4. Phones, browsers, Instagram, YouTube, and WhatsApp all expect MP4.
The AVI is re-encoded to MPEG-4 video and AAC audio in an MP4, with the header placed for streaming. Subtitles and odd extra tracks are not kept.
What you give up by leaving AVI
AVI is an older Windows video container. The video inside may use a codec new players dislike. This site writes MPEG-4 video and MP3 audio inside AVI. It is not a copy of an H.264 stream, and subtitles are dropped.
Keep the MP4. AVI is only for the software that named AVI.
What you gain, and do not gain, with MP4
The download has to be MP4, because that is the format the next step named. Phones, browsers, Instagram, YouTube, and WhatsApp all expect MP4. If the other person can open only one video format, MP4 is that format.
A new encode cannot look cleaner than the MP4 you started with. A remux on a desktop is the lossless path when the streams are already compatible.
A test that matches the real file
Start from the question the other software asked. If it asked for MP4, AVI to MP4 is the matching page. If it asked for something else, switch tools instead of hoping a similar extension will pass. Phones, WhatsApp, and websites reject AVI or play it with no sound. The AVI is re-encoded to MPEG-4 video and AAC audio in an MP4, with the header placed for streaming. Subtitles and odd extra tracks are not kept. The limit is the part people skip, and it is the part that decides whether the download is usable. If FFmpeg cannot read the codec inside the AVI, the conversion fails. A new MP4 cannot repair a damaged or tiny source. The file stays on your device. The first run downloads a converter, so keep this tab open. A phone runs out of memory on long files. A computer is the safer place for a big clip. Check three things in the download: it opens, the AVI material you cared about is still there, and the file size or the layout is what the form described. A new extension alone is not those three things. If a portal, a teacher, or a client rejects the file, send them the original AVI as well when you are allowed to. AVI to MP4 is a convenience copy. It is not a new master. Video files hide the real problem inside the container. AVI to MP4 re-encodes on your device. It does not silently copy every audio language and every subtitle the way a desktop remux can. If you needed those extra tracks, this download is the wrong result even when the picture plays. Convert a one-minute piece before a full movie, and stay on the tab while the converter downloads the first time.
Here is the decision for AVI to MP4. You start with AVI and you stop when a real program accepts MP4. Those old programs get a file they might open. Soft subtitles such as ASS or PGS, and extra audio languages, are awkward or dropped when a tool re-encodes. If those two sentences describe a tradeoff you do not want, do not convert. Renaming the extension by hand will not do this job. Running the tool twice on its own output will not do it either.
If FFmpeg cannot read the codec inside the AVI, the conversion fails. A new MP4 cannot repair a damaged or tiny source.
A related step, if you still need one, is MP4 to AVI.
Other jobs start from the homepage, where each tool is named for the conversion it actually does.