Following the instruction on the Bluetooth doc are not enough to make Bluetooth work on Raspberry pi 4 (the Bluetooth device is not found).

This guide shows how to do it (ignore the Arch ARM specifics)

These are the relevant steps on Void:

• Remove the line dtoverlay=bcmbt (if present) from /boot/config.txt and add the following line to the same file:
  dtparam=krnbt=on
• Make sure that enable_uart=0 is also in /boot/config.txt.
• In /boot/cmdline.txt, remove all references to ttyAMA0