The pattern starts with a number and then subtracts 4 to find the next number. In fact to go from any number to the next we subtract 4.
If we let n = 20, the pattern is 20, 16, 12, 8, 4, ...
If we let n = 40, the pattern is 40, 36, 32, 28, ...
We can use any number for n unless you were given a specific one to start though it did not look that way from the way you wrote the question.