Currently, it's possible to hook up both NEC's, or one NEC and the Thunderbolt display, but I'm stuck at adding the second NEC to the setup. Could it be a hub? OR another cable? Adaptor? Any suggestions?
You're asking to drive a super high resolution display (retina) plus three (3!) other high resolution panels.
Steve is right, that's too much for the machine.
It all may change with external (expensive for sure) thunderbolt-driven VGAs and proper drivers.