23751 different sets of four letters can be formed from the 26 letter of the alphabet, if order is ignored and repetition allowed. (This is the same as the number of ways of selecting four different objects from a group of 26+4-1.) Is it possible, for each set, to find a word containing its letters?
Cohen, Philip M.
"The 4-Set Problem,"
Word Ways: Vol. 16
, Article 2.
Available at: http://digitalcommons.butler.edu/wordways/vol16/iss2/2