To log on to a certain computer account, the user must type in a 4-letter password. In such a password, no letter may be repeated, and only the lower case of a letter may be used. How many such 4-letter passwords are possible? (There are 26 letters in the alphabet.)
26 choices for first letter
25 choices for second letter
24 choices for third letter
23 choices for last letter
26 x 25 x 24 x 23 = ........... choices