hmm
so the hubs are *not* connected in a series/chain, correct?
I haven't tested with more than six erupters myself, so you have me at a bit of a loss here.
Shot in the dark, but have you tried specifying ttyUSB13 & ttyUSB14 even though they don't show with ls?
Thanks for the ideas and feedback.
To answer your question - you are correct, the hubs are not in series, but rather connected in a hub/spoke style. (RPi -> Hub1. Then Hub1 connects to Hub2, 3 and 4 directly.)
I did not try specifying ttyUSB13 & 14, although that is a great suggestion.
I'll try it in a few days when I get back in town, and post my results back here.