I've never changed that O-ring either. Didn't know there was even an O-ring up there. Never took the assembly apart. I just replaced the other O-rings, slid a new fleece filter onto the shaft, made sure it was seated properly and re-installed the filter assembly. Always torqued the cap to proper setting. Would be curious to know what risk we're taking by not changing the #4 O-ring. Does the shaft just pull out of the cap?