How to unlock the 4th world in Plants vs. Zombies 2

Far Future was released on the 27th of March, 2014:

@PlantsVsZombies
The Future is... NOW! Far Future, now available in #PvZ2!

You can unlock it like the other worlds; i.e. paying $4.99 or using a World Key obtained by defeating the 8th level of any of the previously unlocked worlds.

To preemptively answer similar questions about future worlds... If the world in question has a giant padlock on it, then it can be unlocked. Otherwise it will be added in a future release.

@JasonBerkan gave the answer in a comment. The fourth world has not been released yet. So there is currently no way to unlock it.