Question: how many possible combinations of 4 letters?
Answer: reto wrote: How many different 4 letter combinations can be created from the letters A,B,C, and D? This is a case of number of arrangements possible with repetition. Thus you have 4 options each for the 1st,2nd,3rd and 4th letter giving you a total of 4*4*4*4 = 256 number of possible combinations.