If you are on stock 1.9, you just have to follow the steps here : h*tps://topjohnwu.github.io/Magisk/install.html
- grab the zip file in h*tps://images.ecloud.global/stable/FP4/ unzip it and copy boot.img to your phone
- install magisk apk from here : h*tps://github.com/topjohnwu/Magisk
- patch the boot.img file from Magisk app, copy from download to your computer
- install ADB and fastboot on your computer : ht*ps://www.xda-developers.com/install-adb-windows-macos-linux/
- type on a command line " adb reboot bootloader "
- while on bootloader, type " adb flash boot path to your boot_magiskxxxxxx.img" ( the patched file you copied to your computer )
- done
- enjoy and don’t update OTA, this could break Magisk
If you experience brick, you just have to flash back original boot.img
Oh I forget, do you have unlocked bootloader ? If not, go there : https://forum.fairphone.com/t/fp4-bootloader-unlocking/79295
Cheers,