Leaving this one up since there are already responses below, but just a friendly reminder about rule 2 in the sidebar. We ask that posts like these be made on !askandroid@lemdro.id instead. Thanks!

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.

You'll be fine if you read the manual properly before flashing. I've done some very sketchy shit to my Oneplus 8 because I knew I could save it with EDL if I screwed up, but that never ended up happening.

Highly unlikely if you follow the instructions

I don't think you can brick the FP4 that easily, just be careful with TWRP backups, they don't work well with dynamic partitions.

There's a huge warning especially for FP4 on the divestos installation page. Locking the bootloader was problematic on occasion so make sure to check whether it's lockable before you lock it.

The phone's stock operating system is signed with the public test keys from google though, so any security measure regarding that should be relatively easy to circumvent if you know how to sign your own OS.