Another reason to have four (or more than two-three can help as well) is in high MORA areas. The most extreme case I am aware of is near PURPA where the min alt is FL280, just adjacent to K2. I don't believe that a twin can take a commercial payload and sustain FL280 on one engine. We are lucky to fly the route quite frequently and pass over the mountains just after sunrise on the way to Japan, it is quite odd to watch all these majestic peaks rushing past at 450 kts, five thousand feet away, as an aside, we are all waiting for one of our picky training captains (note the small caps) to require us to come back to 250 knots over PURPA!)