I've built Armbain (5.4.17-rockchip64) from sources with additing the next options CONFIG_BT_HCIUART_SERDEV=y, CONFIG_BT_HCIUART_H4=y, CONFIG_BT_HCIUART_BCM=y to linux-rockchip64-current.config.
And bluetooth is working.
brcm_patchram_plus_rk3399[1446]: Done setting baudrate
Feb 3 20:28:32 localhost kernel: [ 99.787745] Bluetooth: Core ver 2.22
Feb 3 20:28:32 localhost kernel: [ 99.787784] NET: Registered protocol family 31
Feb 3 20:28:32 localhost kernel: [ 99.787787] Bluetooth: HCI device and connection manager initialized
Feb 3 20:28:32 localhost kernel: [ 99.787797] Bluetooth: HCI socket layer initialized
Feb 3 20:28:32 localhost kernel: [ 99.787802] Bluetooth: L2CAP socket layer initialized
Feb 3 20:28:32 localhost kernel: [ 99.787809] Bluetooth: SCO socket layer initialized
Feb 3 20:28:32 localhost kernel: [ 99.800667] Bluetooth: HCI UART driver ver 2.3
Feb 3 20:28:32 localhost kernel: [ 99.800672] Bluetooth: HCI UART protocol H4 registered
Feb 3 20:28:32 localhost kernel: [ 99.800829] Bluetooth: HCI UART protocol Broadcom registered
Feb 3 20:28:32 localhost brcm_patchram_plus_rk3399[1446]: Done setting line discpline
Feb 3 20:28:32 localhost systemd[1]: Starting Load/Save RF Kill Switch Status...
Feb 3 20:28:32 localhost systemd[1]: Starting Bluetooth service...
Feb 3 20:28:32 localhost systemd[1]: Started Load/Save RF Kill Switch Status.
rock@rockpi:~$ hciconfig
hci0: Type: Primary Bus: UART
BD Address: 43:45:C5:00:1F:AC ACL MTU: 1021:8 SCO MTU: 64:1
UP RUNNING
RX bytes:1186 acl:0 sco:0 events:77 errors:0
TX bytes:3609 acl:0 sco:0 commands:71 errors:0
rock@rockpi:~$ sudo hcitool lescan
LE Scan ...
D8:2D:27:A6:53:45 (unknown)
D8:2D:27:A6:53:45 Mi Smart Band 4
F8:D8:37:3D:17:75 (unknown)
F8:D8:37:3D:17:75 Mi Smart Band 4
... but only after a cold boot, as @piter75 said.
Who knows what is a right way to make changes in Armbian sources to fix Linux kernel options for Rock Pi 4?
I want to make pull request.