This is because there are four spaces to be filled. The answer is 4. For example a lock combination is in fact a permuation.
If the calculator did not calculate a result please let me know whether you are using a Mac.
For an in-depth explanation of the formulas please visit Combinations and Permutations. Combinations and Permutations Calculator Find out how many different ways to choose items. The answer is 4. Click Kutools Insert List All Combinations see screenshot.