4th Generation Pokémon will be coming to Pokémon Go soon. When it finally releases, there will be 107 new Pokémon to catch. A handful of these will be obtainable by evolving Pokémon from previous Generations. In order to get a head start on completing your Gen 4 Pokédex, you can collect candy for the following Pokémon and choose the best of them to evolve.
I got most of this information from an article on rankedboost.com. My focus was which Pokemon's candy you need to save up to evolve into Gen 4 Pokémon, but their article goes into more depth. Take a look at it to see all of the baby Pokémon and other Gen 4 Pokémon as well as their stats.