Can LibreELEC transcode in web version?

I think your problem is that your RPi4 is capable of HEVC decoding but your tablet and phone are not.

This would be fixed with transcoding but there is a problem, recent Intel/Amd processors with enough power are needed for this job. And the easiest way to implement it would be through a UPnP service (Plex, Jellyfin, ...).

In other words, the answer to your question is NO.

The kodi webinterface is just a "remote" and nothing for playback.

You want something like Jellyfin, there it automatically converts the media that it works at your playback device.

These "transcoding" is rather beefy and the RPi and the other ARM devices do not support it properly.

A rather low end Intel is easily able to do it.