You have to figure out an answer that tells you how many letters are in the answer.
7 wouldn't work because the answer (seven) has 5 letters. The number of letters does not agree with the word.
5 has 4 letters (five) so it doesn't work.

The only number in which the word (the answer) tells you how many letters it has is four.

PS I can figure it out in reverse I would never have gotten it on my own.