Finally, I had a chance to test my Raspberry Pi 4 based on Ubuntu 18.04 under the stars.
So far most things seem to be working with my equipment: Losmandy G11 OnStep mount, SBIG CCD, ZWO174 guider, SX Wheel.
I am currently capturing images with latest Kstars and guiding with PHD2. Astromety plate solving works too.
It looks like the things are faster and I even have some spare memory.
I see that some folks managed to build their RPI4 based on Raspbian, but that approach forces to build all astro software from source. My method allows to use pre-built binaries from Jasem's and Patrick's PPAs.
Given that there is no official Ubunty distribution for RPI4 available, I am using an approach where firmware of Ubunty Server 18.04 is replaced by firmware from Raspbian.
I published more informaiton on setup process on Github:
github.com/avarakin/astropi
Even though the Server flavor of Ununtu is used as a base, the script installs Mate desktop and removes some un-necessary software. The only thing which is not working yet is Wifi but it is low priority for me because this RPI is running observatory, which has Ethernet cable.
Feel free to try this method and let me know if you are facing any issues.
Alex