Dear ALL - thank you for all info provided! @NicoD link helped to run 5.8 Debian CLI version (on SD) first and easily update and upgrade. The only problem I have noticed is HDMI signal quality (?) - not as bad as in case of Diet version for RockPi4 - but still not usable. Screen "shakes" and it is not possible to work with (however it looks that system settings are OK for my Samsung screen). The good thing is that it is possile to set basic configuration (i.e. WiFi in my case) and go to SSH. My RockPi4 is planned for headless jobs anyway, thus I have to say - IT WORKS!! THANX!
It is my first RockPi 4 board and first Armbian install - board working OK with official Radxa image (Debian 9) and eMMC drive. Since my plan is to use just CLI I'm trying to install Armbian Buster images. Unforunately both "current" and "legacy" versions are not booting up. I have used SDFormatter (just in case) and Balena (as usual) on 16B Transcend Premim and 32GB SanDisk Ultra (A1) cards - same result: board is powered on but nothing happened at all. Only green diode is lighting but no signs from SD card (red diode is not showing up). eMMC card not connected in both cases. I have checked other topics (i.e. ROCK64) but can not find a good solution how to proceed - your help is much appreciated.