Ok, tested with all five in Dalaran, instead of each instance of Wow using 390MB of memory, it was using 580MB. That's it. Sooo...I never went over 3.6GB of memory used with all five dancing in Dalaran during a peak evening period.

If you crank your graphics, you'll use more, one client on full will use 1-1.5GB of memory, but that's on FULL settings at full res. If that's how you run things, you need gobs of RAM. If you run smaller child instances, you simply do not. 4GB is fine for 5 clients, 4 on low.