

Postmarket is available for the FP4 but seems to be not really usable judging from the table:
https://wiki.postmarketos.org/wiki/Devices
I’m running Calyx (😭) on the FP5 atm, unlocking the bootloader seems to work for FP4 - 6:
https://www.fairphone.com/en/bootloader-unlocking-code-for-fairphone
Looks like Ubuntu Touch is also working on the FP4:
https://devices.ubuntu-touch.io/device/fp4/