I have been using Openhab since version 2.4 on a Pi3B for quite some time. Recently built new on a Pi4 with OH 3.4.
I installed Azul Java 17 with the Openhab 3.4 installation, no problems with that even though it wasn’t called for until OH4.
Running Debian Bullseye Linux 11 on the pi4 (4GB memory) with a Argon40 case with integrated SSD that is bootable.
I was pleasantly surprised that running
sudo apt update sudo apt upgrade
led me to upgrading me from OH 3.4 to 4.0.2 and auto ran the conversion bash script(s)
I did have to upgrade the GPG signing key, but the one liner provided in the 4.0 installation notes was self explanatory.
All existing things, Items, models, UI rules, pages all run fine. Had a few things to cleanup in the OHL.
Would like to congratulate the dev team on this upgrade accomplishment for OpenHab.