The simple answer is 24. You multiply the 4 x 6. Here's another for you:
I have 1 red shirt, 1 blue shirt and 1 black shirt. I also have 1 pair of white pants, 1 pair of blue pants, 1 pair of green pants and 1 pair of black pants. HOw many different outfits can I make from the 3 shirts and 4 pair of pants?