I tried as well but found no guides to build a RPi4 LinuxCNC machine with realtime. Can you point me to some tutorial or ready SD card image?

I'll go through my saved links tonight. I remember it was a bit of a cludgy operation, like all things new+Linux, but after I got everything setup it all worked as expected.

I think this forum post pointed me in the right direction: https://www.forum.linuxcnc.org/18-computer/36879-raspberry-p...