STUMPERS XLVII

The following riddle seems unanswerable, since no riddle is given. But there is one answer - and only one.

How many letters are in the answer to this riddle?

Answer:

The answer must be FOUR - the only number word that contains the same number of letters as its name. 

NOTE from JA: Another way of looking at this is to say, "How many letters are in the (answer) word FOUR - and the answer is four.