Adding Bluetooth support to ArchLinux ARM's Raspberry Pi 4 version

90-pi-bluetooth.rules

# /lib/udev/rules.d/90-pi-bluetooth.rules
# Copied from https://github.com/RPi-Distro/pi-bluetooth/blob/master/lib/udev/rules.d/90-pi-bluetooth.rules

# Raspberry Pi bluetooth module: enable routing of SCO packets to the HCI interface
ACTION=="add", SUBSYSTEM=="bluetooth", KERNEL=="hci[0-9]", TAG+="systemd", ENV{SYSTEMD_WANTS}+="bthelper@%k.service"

99-serial.rules

# /etc/udev/rules.d/99-serial.rules
# Copied from Raspbian's 99-com.rules

KERNEL=="ttyAMA[01]", PROGRAM="/bin/sh -c '\
	ALIASES=/proc/device-tree/aliases; \
	if cmp -s $ALIASES/uart0 $ALIASES/serial0; then \
		echo 0;\
	elif cmp -s $ALIASES/uart0 $ALIASES/serial1; then \
		echo 1; \
	else \
		exit 1; \
	fi\
'", SYMLINK+="serial%c"

KERNEL=="ttyS0", PROGRAM="/bin/sh -c '\
	ALIASES=/proc/device-tree/aliases; \
	if cmp -s $ALIASES/uart1 $ALIASES/serial0; then \
		echo 0; \
	elif cmp -s $ALIASES/uart1 $ALIASES/serial1; then \
		echo 1; \
	else \
		exit 1; \
	fi \
'", SYMLINK+="serial%c"

btuart.sh

#!/bin/sh
# /usr/bin/btuart
# Based on https://github.com/RPi-Distro/pi-bluetooth/blob/master/usr/bin/btuart

HCIATTACH=/usr/bin/btattach
if grep -q "Pi 4" /proc/device-tree/model; then
  BDADDR=
else
  SERIAL=`cat /proc/device-tree/serial-number | cut -c9-`
  B1=`echo $SERIAL | cut -c3-4`
  B2=`echo $SERIAL | cut -c5-6`
  B3=`echo $SERIAL | cut -c7-8`
  BDADDR=`printf b8:27:eb:%02x:%02x:%02x $((0x$B1 ^ 0xaa)) $((0x$B2 ^ 0xaa)) $((0x$B3 ^ 0xaa))`
fi

uart0="`cat /proc/device-tree/aliases/uart0`"
serial1="`cat /proc/device-tree/aliases/serial1`"

if [ "$uart0" = "$serial1" ] ; then
	uart0_pins="`wc -c /proc/device-tree/soc/gpio@7e200000/uart0_pins/brcm\,pins | cut -f 1 -d ' '`"
	if [ "$uart0_pins" = "16" ] ; then
		$HCIATTACH -B /dev/serial1 -P bcm -S 3000000
	else
		$HCIATTACH -B /dev/serial1 -P bcm -S 921600 --noflowctl
	fi
else
	$HCIATTACH -B /dev/serial1 -P bcm -S 460800 --noflowctl
fi

hciuart.service

# /lib/systemd/system/hciuart.service
# Copied from Raspbian

[Unit]
Description=Configure Bluetooth Modems connected by UART
ConditionFileNotEmpty=/proc/device-tree/soc/gpio@7e200000/bt_pins/brcm,pins
Requires=dev-serial1.device
After=dev-serial1.device

[Service]
Type=simple
ExecStart=/usr/bin/btuart

[Install]
WantedBy=multi-user.target

Thanks for your efforts!
Linking back to the forum (for future visitors): https://archlinuxarm.org/forum/viewtopic.php?t=14244