Return 4th highest number not in list.

From a given list of numbers I want to return the 4th highest number that is not in the list.

If a given list is 1, 2, 3 then the next highest number is 4 but if the list were 1, 6, 3 the 4th highest number not listed is 2. I'm not sure how to accomplish this. A list of 1, 2, 7 would return 3.

Jim O