how many combinations of 4 numbers out of 6?
So there are possible numbers of length 4. It's easy. The first digit can be 1,2,3,4,5 or 6 (6 combinations). We choose the second,third and fourth digit the same way we did it with the first (so it's 6*6*6 combinations) and the final answer is 6*6*6*6=1296 combinations.