You should install Security patches immediately when available, given Fairphone waits a month or two before releasing them, giving hackers a comfortable head start… Security patches are different from other updates, see my detailed explanation here.
As for the Android 15 (not 14) update, I would wait before getting it, because it has currently two rather nasty bugs: Firstly random reboots, and secondly a Launcher problem specifically for those living or having visited France, the Netherlands and some other European countries. You sound French (“AZERTY”, “septembre” 
), so you would be hit with both barrels…
Unfortunately the November Security patches for the FP4 are inside that buggy A15 update, which means FP4 owners have the choice of either risking an unusable phone, or risking to be shut out of banking and corporate VPNs. What one calls “being stuck between a rock and a hard place”… 