A painter is going to paint a room shaped like a cube, with square walls that are 20 ft. on each side. A gallon of paint can be used to cover 400 square feet. How many gallons of paint should the painter buy if he is going to paint the four walls of the room?
Each wall has an area of 20 ft. * 20 ft. = <<20*20=400>>400 square feet.
Thus, the four walls altogether will have an area of 4 * 400 square feet = <<4*400=1600>>1600 square feet.
Therefore, the painter should buy 1600 square feet / 400 square feet/gallon = <<1600/400=4>>4 gallons of paint. Answer: \boxed{4}.