Are you sure you got the question right?
I can see a combination which is not on your list (ignoring digit swap only):
(ones) 1, 2, 3, 4
(twos) 12, 13, 14, 23, 24, 34
(threes) 123, 124, 134, 234
(fours) 1234
Your list does not contain a "134" - could this be why you are having trouble doing it for yourself?
There are a lot of ways to do it: see
here[
^] and
here[
^] for discussions with at the very least pseudo code, or of course there is good old Google:
http://www.google.co.uk/search?sourceid=chrome&ie=UTF-8&q=Generate+all+combinations+of+four+characters+c%23[
^]