isolated 'em after you measured more than 3 per 100 eh?
and only one if four years, sounds like you have some pretty good bees there.
in that one hive with the bigger load, did you notice any other problems? was it weaker in numbers than your others, not drawing as much comb, not storing as much honey? any sick brood?
i have a lot of 'feral' unmanaged colonies all around my area, (my county is 68% wooded), so i don't have any place i would feel comfortable removing an infested colony to.
that is why i'm leaning toward killing the queen, letting most of the brood emerge, dusting them off, and starting them over.