Follow the install instructions at the bottom of https://archlinuxarm.org/platforms/armv8/broadcom/raspberry-pi-4#installation and boot.
Install linux-rpi as kernel and verify /boot/cmdline.txt to point to the right root device before rebooting.
The RPI4 supports booting from USB out of the box.
Refer to https://www.raspberrypi.com/documentation/computers/raspberry-pi.html#usb-boot-modes
Most documentation assumes booting from mmcblk devices. Make sure you always point to the right devices or partitions, particularly in /boot/cmdline.txt and /etc/fstab.
The RPI4 has issues with a high IO load when running in 32-bit mode. Running AArch64 solves these. If you experience the oom-killer kicking in with no obvious memory hungry apps but high IO, then switch to AArch64.
Sources:
- raspberrypi/linux#3210 (comment) contains the hint to not use 32-bit.
- https://forums.raspberrypi.com/viewtopic.php?t=249924
- https://forums.raspberrypi.com/viewtopic.php?t=67204