i built a kernel for the rpi4 on raspbian buster that includes the rt-preempt patches  (setup as fully preemptible) and the usb lowlatency patch . You can find it here:
http://fps.io/raspberrypi4-linux-4.19.6 ... ncy.tar.gz
I hope I included all the nessecary things. This was my tar command for gathering the parts of the installed kernel:
sudo tar czvf linux-4.19.65-rt24-v7l+-usb-lowlatency.tar.gz lib/modules/4.19.65-rt24-v7l+/ boot/*.dtb /boot/overlays/ /boot/kernel7l.img
To install it you should be able to just change to the root directory of your filesystem and just untar it:
sudo tar xzf raspberrypi4-linux-4.19.65-rt24-v7l+-usb-lowlatency.tar.gz
and then reboot [possibly you need to run a depmod, not sure].
With this kernel I am running a Focusrite 2i2 (a usb 2.0 class audio device) succesfully with zero x-runs for the last couple of hours with a period size of 64 frames at 48khz sampling rate on a tuned system  and running a guitar effects patch in mod-host.
 https://mirrors.edge.kernel.org/pub/lin ... 4.patch.xz
 https://aur.archlinux.org/cgit/aur.git/ ... tency-dkms
 using the rtirq package with:
RTIRQ_NAME_LIST="snd usb i8042"
in /etc/defaults/rtirq to setup the prioritoes. jackd is running at prio 80. I hope this is useful for any other people that finally get to use a raspberry pi4 as realtime audio effects processor using a USB sound card..