A company's computer must consist of 4 letters with no repetition. How many passwords are possible?

Assuming that the password is letters only, and that upper versus lower case doesn't matter, you have 26 letters to choose from. So your first letter can be any one of 26. For the second letter you are not allowed to repeat the one you chose for the first, so there are 25 choices remaining. For the third letter that leaves 24 choices. Thus the number of ways you can have a three-letter password with no letters repeating is 26 x 25 x 24. Can you continue this line of reasoning to determine how many 4-letter passwords are possible?