If you are using them with a DS7000 system, then as long as all the Mini Powerhorns are on the same house code as the DS7000 there really isn't a limit since they are only receivers looking for a repetition of All Lights ON / All Light OFF on the power line.
Technically, they can all be set to the same unit code also. I'm not really sure why they even have a Unit code setting? Maybe one of the more experienced users can answer that one.
Since you have 1 working, I would set the other 3 to the same house & unit codes and test them one at a time in the same outlet to verify they all work. If they do, then you most likely have a phase and/or noise issue with the other outlets you tried.