Convert TMP to MP4

It is very unlikely that you can convert random TMP 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 TMP file should contain fragments of multimedia data, it is still highly unlikely that you will be able to convert TMP to MP4.

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

In a word, direct tmp to mp4 conversion is not possible.
So don't expect any so-called tmp to mp4 converter or tmp to mp4 free online conversion tool available.