Or, heck there's another way you could do it without a piece of paper by counting on your fingers:

Look at your current HP number. Wait for him to tell you what he wants. Count on your fingers and ask yourself each time "is this divisible?"

Example: You get 1 and he asks for a multiple of 4.

1: Is this divisible by 4? no.
2: Is this divisible by 4? no. (holding 1 finger up)
3: Is this divisible by 4? no. (holding 2 fingers up)
4: Is this divisible by 4? Yes. (holding 3 fingers up)

Stand on 3.

Given the fact that there are only 4 circles to stand on, you only need to do this a maximum of 5 times, and you get roughly 2 seconds per calculation.