I suggest you use the "Search" function at the top left with the keywords "permutations" or "combinations" as this will turn up lots of code to give you a head start. Broadly speaking you want to assign each letter to an index in an array and jumble them up in all possible ways, which is a standard algorithm.
If you want the maximum number of combinations of four prechosen letters where each letter is only used once, I think that there are 24 combinations btw (4!) and if any letter may be used more than once, there are something like 456,976 combinations (26^4)
UPDATE: Thanks to pinetree for the link to the maths faq. In the above node, I am talking about permutations and NOT combinations. You live and learn.
Please, if this node offends you, re-read it. Think for a bit. I am almost certainly not trying to offend you. Remember - Please never take anything I do or say seriously.