Added RPI4 install instructions

Raspberry_Pi_4.md

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+## Kernel
+
+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.
+
+## Boot from USB drive
+
+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.
+
+## Prefer AArch64 for high IO loads and out-of-memory issues
+
+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:
+
+- <https://github.com/raspberrypi/linux/issues/3210#issuecomment-716510916>
+ contains the hint to not use 32-bit.
+- <https://forums.raspberrypi.com/viewtopic.php?t=249924>
+- <https://forums.raspberrypi.com/viewtopic.php?t=67204>