Convert TEMP to MP4

It is very unlikely that you can convert random TEMP files to MP4 format, unless it was some renamed multimedia file, or a fragment of it, in which case it might be possible.

Temporary files are generated for many purposes, and even if a TEMP file should contain fragments of multimedia data, it is still highly unlikely that you will be able to convert TEMP to MP4.

You can always try renaming your file's suffix from TEMP to MP4, and try to play it using a common media player, but the chances are pretty low.

Simply put, it is impossible to convert TEMP to MP4 directly.
Thus, there is no so-called temp to mp4 converter or a free online temp to mp4 converter to be found.