Board Thread:Questions and Answers/@comment-32306613-20170616162304/@comment-26155909-20170616175858

You unlock new fleets with quests.

- fleet 2 with quest A4: Have 6 ships in your fleet

- fleet 3 with quest A14: Have Sendai, Jintsuu, and Naka in your main fleet

- fleet 4 with quest A16: Have Kongou, Hiei, Haruna, and Kirishima in your main fleet

Check this page for more info on quests: http://kancolle.wikia.com/wiki/Quests