I can't even remember where it was originally said (I think in a Live Letter?) by Yoshi-P that 4.0 was going to be a separate storyline from Heavensward (but still based off the 2.0 main story) and that you wouldn't need to own Heavensward to play 4.0. That's as much as I know that has been officially stated.

Belhi is right though, major patches normally around every 3.5 months, and 2.0 went until 2.5 but the devs said if they feel they need to expand to 3.6+ then they will.