OK, after reviewing and THINKING about your responses, I will have to back down but under conditions on this one. There is nothing that would required a 4th wire, However: 1) If you ran 2/2/4 SER cable to the sub panel then the 4awg wire would have to go to the equipment grounding terminal bar. Nothing can be connected to the neutral (grounded conductor) bar. This way in the future, connecting a 120vac circuit would not work. 2) The bonding screw for the neutral bar would have to be left out. 3) As an AHJ I would want a sticker on the panel stating 240vac loads only on 240vac dbl pole breakers. So you are right, nothing in the NEC prohibits this but it all depends on how you make the connections in the panel.