I'm interested in buying Raspberry Pi 4. I'd like to create simple server for testing my applications and storing data (home cloud). I wonder if I can manually install Ubuntu Server like on standard computer. Can I boot from USB and install OS on SD card / external disk instead of burning preinstalled Raspbian / Ubuntu?
The simplest way is to visit the download site and select your version. I would go for the 64bit 20.04 for your proposed Pi 4B and drop it onto an SD card for now.
Play around for a few weeks till the USB SSD/HDD boot is a bit more complete or help out and test the beta software.
I have one 20.04 Intel box running with around 8Tb of data so far as a temporary NAS and not broken it yet :)