Well, it's Deja vu time and I am just seeing the latest replies to this thread that I had started 4 years ago!
FWIW: I had solved the problem by adding more transformers and distributing the load to each, however I am in the same situation yet again. I suppose its my own fault, as I continue to add buildings with lights here and there around the layout thinking that one or two won't exceed limits, silly and hopeful me (think of the father in A Christmas Story where he has myriad Christmas lights plugged in the wall outlet). The amperage output per Z-4000 "handle" or side is between 5-6 amps, and given that I have two Z-4000s just for accessories/buildings that totals about 11-12 amps per transformer, which isn't good apparently. I guess I should've bought a couple of ZWs a while back when I had the chance, but at that time was broke. I will likely need to add yet another transformer just for accessory lighting! It is getting monotonous but building a fantasy world knows no bounds.