Well, that out-of-memory popup makes perfect sense if 4 work but adding the 5th causes FPS issues. I would review what other programs are running. Chrome is well known for sucking up all available resources for its own ends. I ran 5 in MoP at 1920x1080 with a GTX460 and 12GB of RAM and would occasionally see the OOM warning. You can drop the renderer back from DX11 to DX9 to free up ~15% of client memory which will stave off the OOM warnings for a bit but it should still gripe after several hours of play as the clients grow.

What's different about the "master" account?