who currently makes / sells swarm style subwoofer systems?

The AK system is a passive system with a centralized amp, but I imagine you could do the same with just about any set of 4 active subs, instead.

@audiocanada is completely right. In fact, some people prefer to have four separate amplifiers to tweak the phase of each subwoofer based on location. So you would be fine with four independent subwoofers.

James Romeyn Music and Audio offers systems based on the asymmetrically distributed multi-sub concept.  

AudioKinesis has, at times, worked with customers to find semi-custom solutions if the "off the shelf" one isn't a good fit.

And of course the basic concept can be implemented in multiple ways, as has been mentioned. 

Duke