I ran a pair of Labs outdoors, half space, I ran DR250's on top, very efficient horn loaded design. The sound was balanced with recorded music, I think we played some Tool at full power. The 250 needed everything from the pair of Labs. I know with 4 Labs the output is much greater, but a pair is less impresssive outdoors. Still better than a pair of dual 18's, more powerful and much cleaner with better extension.
When I mentioned wide variety of drivers for the Tuba36, I mean to say there are a few very important parameters that matter, Fs, Xmax, Xmech, Bl, the rest are less important and make little or no difference. This allows me to get the same performance from a pair of 15" Audiopile drivers as the Emminence Magnum, up to a point, the Magnum will be take much more abuse, but you pay three times the price. Depends totally on your application.
If I were building more Labs for heavy outdoor use, I would use a minimum of 4.