riddle - What is the solution?

What is a word made up of 4 letters,

yet is also made up of 3,

Sometimes is written with 9 letters, and then with 4,

Rarely consists of 6,

and never is written with 5 ?

Answer

Can I say that:

The question is "What is the solution?"

And following the pattern in the clue (each number describes the number of letters in the first word in the phrase), then:

The solution is a phrase made up of 11 letters

(space is not a letter)