How many ways to combine 4 numbers?

Hi,

If you have a set, {a,b,c,d) how to you calculate the number of different combinations, and what are they?

I thought it was 4^2 = 16 but now I am not so sure...

Thanks.
Update: I'm talking order, so and I'm using [1,2,3,4]

[1234], [1243], [1324], [1342], [1423], [1432], [2134], [2143], [2314], [2341], [2413], [2431], [3124], [3142], [3241], [3214], [4123], [4132], [4213], [4231], [4312], [4321]

two missing?
Update 2: There are 4! = 24 different orders they can be put in (i.e. abcd, bcad, dcba, etc.).
Update 3: This was most helpful:

http://www.analyzemath.com/statistics/permutations_combinations.html
6 answers 6