Take it twenty-six, cut it down to four bars, make it a hook
If it's not I'm sure to send a book

from GZA – Knock, Knock Lyrics on Rap Genius

Meaning

If a song by GZA doesn’t have a hook, that means he’s compromising a hook for more in-depth lyrics. (i.e. — GZA will send you a book of knowledge in his song)

Also, I think he’s saying to other rappers who go on and on about nothing: take all those useless bars, and maybe your four best will be good enough for a hook, which usually isn’t as important or have as much substance as the actual verse’s of a song. Basically he’s belittling others' lyrics to practically nothing.

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