Arch linux ARM Bluetooth firmware support for Raspberry Pi's include Raspberry Pi 4. It supports aarch64 as well.
Clone repository into working directory and use makepkg in standard way, i.e. something like that:
cd ~
git clone https://github.com/WTFThatPerson/bluetooth-rbp.git
cd bluetooth-rbp
makepkg -si
sudo reboot
sudo systemctl start hciuart
sudo systemctl enable hciuart
Now You're ready to install bluez and bluez-utils and start using Bluetooth on Your device
Please keep in mind, since kernel 5.4.50 this isn't needed anymore neither on Pi3 nor Pi4 because of native support of Bluetooth: just put
dtparam=krnbt=on
in /boot/config.txt and install bluez and bluez-utils is enough now