Full guide can be found at http://www.simplerobot.net/2019/12/xenomai-3-for-raspberry-pi-4.html
scripts, guide, pre-built kernel with xenomai 3 for raspberry pi 4
built 4.19.y ipipe patched kernel + prebuilt xenomai user-space libraries and tool. Pull down and deploy
if you just want to have READY-TO_USE realtime kernel, get this