http://www.amazon.com/gp/product/B005KN1YV6/
You can also use a wireless switch. The problem with PS3 wireless is that it's crap and unreliable; Plugging the four PS3s into this switch bounds them to it, but it makes wireless viable because now you're depending on the switch's wireless capabilities. I've used it for a few hundred games. Offline Full Boost is fun as hell.