As far as I know, there isn't any particular usage pattern in most cases... often, they explore with all their arms at once. For walking behaviors and diving onto crabs, they use their arms in a coordinated way, but they're still mostly doing the same things a lot of the time. I'm not sure if they have a preference for behaviors that aren't symmetric; it seems like they often grab hold of something solid with the back 4 arms and use the front ones for tug-of-war, and they might use the front arms more often for one-arm strikes on small prey. I suspect that the "walking coconut impersonation" that aculeatus does always uses the same arm pair, but Mucktopus would be the person to ask for sure on that one.
Mucktopus mentions in the paper noted on the home page that the enlarged suckers were found on only some of the arms of the species she was recording. My two merc males that are easily observed (albeit under red light only) have enlarged suckers on all eight arms. I mentioned this to Crissy and she said that it is more common to see them on two or four but not all eight arms BUT they still don't have a definitive answer as to their use.