A painter is going to paint a room shaped like a cube with square walls that are 20 feet 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
The total surface area of the cube-shaped room is 6 * (20 ft * 20 ft) = 2400 square feet.
Since the four walls will be painted, the total area to be painted is 4 * (20 ft * 20 ft) = 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}.