Right now I have two full camps, a full city, and a full army. When I try to transfer a unit from one army to another it's impossible to switch between two armies. For example, when I want to transfer my hero into my full town, I have to create a new army and transfer a unit out by itself, then transfer the hero into the town after. When I take the hero back out I have to reverse it all over again.
I basically don't get to have a fifth army because of this flaw. If I ever wanted to build a fifth full army, I wouldn't be able to transfer any units at all and my hero would be stuck outside of his town forever unless I start discarding units.