A sort of surprise! It has been a few days since files neccessary for the update appeared in game’s Android version files, but nobody expected that it will come out that fast. Well, it did, and both Android and iPhone players can now enjoy catching new pokemon. Some of them, at least: update will come out in portions. Some of the new monsters available now are Bidoof, Turtwig, Chimchar and Buneary. Niantic has also implemented new CP-counting algorithms, but these were undone almsost immediately. We can expect them to be brought back in any day.
Trailer of the 4th gen update can be watched here.