If your speedlights are chewing through a ton of AA batteries or you have a long shoot and can’t afford the time it takes to swap out a fresh set, then consider using a Jackrabbit II battery pack from Dyna-lite. One pack can simultaneously power two SB-900 Speedlights and will last all day. They also work with the thermal circuit. They work particularly well with the FourSquare where you would only use two Jackrabbits to power all four Strobes.
You can find the Dyna-lite power packs at http://www.dynalite.com/battery_power.shtml
Makes for a neat travel kit and you can pack it all up in a Lightware 2012. Easy carry on size. You know at a quick glance you have what you need for the shoot.