What has $4$ letters, sometimes $9$ letters, always $6$ letters and never $5$ letters. How?
Hint: $4$ letters
Well; for a start:
The word "what" itself has four letters, the word "sometimes" has nine, the word "always" has six letters and the word "never" has five. So...
What has $4$ letters, sometimes $9$ letters, always $6$ letters and never $5$ letters. How? $3$ letters!
* worth noting that this was entirely @WAF in the comments.
I kind of second Hugh's answer, but with a slight addition. What's in the question's body isn't really a question. It's a statement. What has 4 letters, sometimes 9 letters, always 6 letters and never 5 letters. It's just a count of the number of letters in each word succeeding the comma. This is like a guide/legend to answer the real question.
The real question
lies in the title ==> But, how is this possible?
Taking the prototype in the statement given above... this has 4 letters. That is how THIS is possible. In other words... the answer to How is THIS possible?