SD, if you happened to have a spare VP100, it certainly could be tried in back, preferably oriented vertically. Buying one just for that purpose isn't a good idea in my view. The short distance of 4' behind you is marginal for setting up a rear sound field, and speakers of very wide dispersion, such as the QSs, would be advisable at that distance.
Note that actual discrete 6 or 7 channel material isn't necessary to benefit from rear surrounds, since processing such as DPLIIx can be applied to 2 and 5 channel source material to send appropriate back ambience to them.
