4^3.
Unlike many probability questions on these tests, in this case, each letter is completely independent of the other letters. This is unlike, say, a seating arrangement, since if one seat is filled, the seat is no longer an option for the others.
Since each letter can go into one of the four mailboxes, there are 4 * 4 * 4 possibilities. So, 4^3.
You should probably at least understand the meaning of "4 pick 3" and "4 choose 3", though. They pop up occasionally.
I tutor GMAT/GRE level mathematics privately in the Los Angeles region, as well as via Skype for a discounted rate. Send me a message if you're interested.
