Well, i remember reading that the third engine doesn't work anymore because, nowadays, 2 engined jets are just fine, and burn less gas. they are also very reliable. I think there was a debate not too long ago as to weather ETOPS saved an airline money or cost them.
The four engined jets are also good, because some planes need those extra engines, and the 4 engines allow the plane to have a higher cruising speed, and the extra 2 engines save wear and tear, since they don't have to run the engines as high.
Anyway, I read this on a post a couple of months ago, so I'm just guessing.