Acoustat did offer their passive "cube" sub as a stand alone component, though it was primarily meant for use with the model One, and the others in the "line source" series, such as the 1+1, & 2+2. I can't see much reason to use one on Fours... they produce much more bass, both in terms of volume as well as depth, than those little passive cone subs could ever hope to! In my experience, there is little need for any additional bass on any of the Acoustats with more than 2 panels.
Actually, IME model 4s biggest down side is how they tend towards over powering bass in most normal sized rooms. Because of that, I would suggest you only consider them if you have a pretty large listening room, and don't mind pulling them well out from the back wall. Tube traps, or similar can go a long way in helping calm their bass as well. It's not as though they'll sound awful, just a bit too "warm" to be considered really accurate.
On the up side, with Acoustats, the more panel area the have, the easier they are to drive. I know it sounds counter-intuitive, but because the panels don't have to be driven as hard to energize the room, they don't ask as much of your amplifier. Also, the larger panels present a less reactive load, so don't tend to be "amp eaters" like 1+1s, in particular, can be. I've even run 4s off modest tube amps - though I think they do much better on good solid state (100+wpc), particularly as they do tent towards the warm/bass heavy side anyway, most tube amps only make that worse.
Over all, they are wonderful speakers, and as long as you have a big enough room, I heartily recommend you give them a try... once you go "stat, you'll never look back!
PS, Acoustats are particularly robust, while ime Carvers?... well not so much! You are much more likely to spend time repairing Carver (or Apogee) ribbons than ever needing to fiddle with an Acoustat panel. I'd MUCH rather freshen up power supplies on Acoustats (if needed) than try to source new ribbons
