How many K-On! manga libros are there to the series?. And when should i read K-On! College Manga?.

I really wanna know cause i am almost finished with K-On! manga vol 3. I am on the last chapter of K-On! manga vol 3. And after i read K-On! vol 4 should i than start lectura K-On! College?. Please tell me cause i really need to know.