Tvheadend

Are you wanting to transcode or remux?

Remuxing is not a problem, I post-process my .ts files to .mkv without problems.
Transcoding would be a matter of having enough memory & patience, and the practicality would depend on what formats you wanted to go to/from.
A Pi4 should be able to do it OK, though you might want some swap to cope with situations when more than 1 transcoding was happening at once [or script the postprocessing so the jobs were serialised].

Is Tvheadend having out of the box support for remux? I didn't find. Do you need to use the post-processing scripts to do that? Can you easily remux in a way that you replace the larger file with smaller and it would be still visible and available in the Tvheadend.

La Te wrote:

Is Tvheadend having out of the box support for remux?

TVH provides an mkv recording profile out of the box, but I have found with some IPTV streams this would have problems if there was an error at the start of a recording.

I didn't find. Do you need to use the post-processing scripts to do that?

I can be done with post-processing.

Can you easily remux in a way that you replace the larger file with smaller and it would be still visible and available in the Tvheadend.

Your post-processed script can be visible & available in TVH however remuxing isn't going to significantly change the file size, you need transcoding for that.