For a number to be divisible by 4 you just need to worry about the 10s and 1s place. You also want to know that the 1s cylce every 20, or in other words:
4 8 12 16 20
24 28 32 36 40 and so on. So the only numbers that x can be there are 1, 3, 5, 7, 9. Does that make sense? and can you manage the rest?
