Why is the accept / utility of one of my miners consistently higher than the others?
It appears that the worker with the more shares/utility is solving low difficulty pool shares, where the other 3 are probably mining at a more appropriate difficulty level. 

Utility is the measure of how many shares are submitted per minute (diff1*pool difficulty mining at). That suggests further that the unit is working on lower difficulty shares than the other workers. 

Usually on your pool website there will be a spot to change worker difficulty, and for those units, I would recommend 2 for the ICA and 8 for AMU, but you could always adjust according to your preference.