http://www.newsweek.com/pokemon-go-gen-4-update-what-catch-1172599

Gen 4 in Pokémon Go is finally live, allowing trainers to catch new and different Pokémon to trade, battle and collect.

However, like the introduction of previous generations in Pokémon Go, not every Pokémon from the region will be available on Day One. With the Sinnoh region, expect to see some of the “early route” Pokémon to appear as well as the Starters.