How do you make a 747 go faster? Simple: just stick another engine on it and add a button that says "turbo" in big red letters. YEEEAAAHHH!!!
Alright, well, now that we've got your attention, we've got to admit that while this fifth engine might possibly add a little bit of extra thrust, that's not what Pratt & Whitney set it up for. The engine in question is a PurePower PW1200G, which is intended for the Mitsubishi Regional Jet, a 70-90 seat passenger aircraft that Japan is building for 2015. Part of the test regime involves flying the PW1200G at altitude, and normally, P&W would just swap out one of the 747's own engines for the new one. But, the PW1200G is too small for this, which necessitated the custom test pylon on top of the 747.
P&W is saying that the PurePower series of engines offers "double-digit improvements in fuel burn, environmental emissions, noise and operating costs thanks to a "game-changing geared turbofan," which sounds pretty fancy. But, we're still holding out hope that someone'll stick a bunch more engines on a 747 just to see if they can kick it up past the sound barrier or something.