OS other than Android ?
The one i personally know is PostmarketOS based on Alpine Linux,
but also “Ubuntu Touch”, “mobian” and “SailfishOS” are possible to install, also linux based…
.
List of ✏ Operating Systems for Fairphones
.
the others supported OS listed there for the FP4 are all Android…
“FairphoneOS”, “LineageOS”, “/e/OS”, “iodéOS”, “LeOS”, “CalyxOS”,
(as business partners, both Fairphone and Murena sell devices with /e/OS preinstalled and relocked bootloader),