That’s what some people said online but yes there is ideed arb on it, there are some technical details on the /e/OS FP4 page for instance : https://doc.e.foundation/devices/FP4/install.
I was using Calyx and went to Divest with relative ease, atlhough I did pay attention to the security patch level before jumping, having to send the phone to fairphone would be really annoying.
Just be careful if you test several android roms, the phone uses anti rollback (ARB), like pixel phones. In certain circumstances it’s possible to brick the phone, and you’ll have to send it to fairphone.
With android roms that allow relocking the fairphone bootloader (Calyx and Divest for instance), really check if the bootloader can be relocked via fastboot command-line, it will brick if it’s not the case.
I spent some time reading before attempting anything on it and it went rather smoothly, I don’t hop a lot between roms though.
This discussion about bricking the phone after a locking attempt is interesting, also mentioned on the Divest wiki : https://forum.fairphone.com/t/trapped-in-fastboot-mode-with-locked-bootloader-and-corrupted-custom-rom/80985/280
I just used fastboot to get the locking ability, the result was 1 so it could be locked, I did and it went ok. No problem since.