Update November 14th, 2018: Some Generation 4 evolutions are out now, and can be evolved with a "Sinnoh Stone" you get from completing a week's worth of research quests. Eevee evolutions, along with Gallade and a few others, are not yet available.
Pokémon Go is back and better than ever. While many people left the game due to the server issues and lack of content, those issues are a thing of the past. Now there are quests, monthly events, and the new Friendship system to keep trainers occupied. There's also a new batch of Pokémon on the horizon. Generation four, with favorites like Lucario and Garchomp, has been hinted by Niantic.
While Chimchar climbing pole is an exciting tease for new Pokémon, the fourth generation is also noteworthy for giving powerful evolutions to old friends. Magmar and Electabuzz take a step further. Porygon gets weirder. And the ever-popular Eevee gets some new members added to its family tree. Overall, there are 22 old Pokémon that get new evolutions in gen four.
That's a lot of work to do. To make it easier, we've narrowed it down to the gen 4 Pokémon that are strongest and most popular. These are the evolutions you'll want to get ready for, and the Pokémon they evolve from.
Rhydon is already a go-to for raid battles and gym defending. So many legendary Pokémon having Flying or Fire typing, so the sturdy Rock/Ground type isn't losing its usefulness anytime soon. With its gen four evolution into Rhyperior, it will only prove more essential. Projections by knowledgeable websites like Pokémon Go Hub put Rhyperior at a possible max CP of 3869. There are few non-legendary Pokémon that pack that kind of punch.
Eevee is one of the most beloved Pokémon of all time. It's even starring alongside Pikachu in the next game! As such, any new evolution for Eevee is bound to create buzz. The Grass type Leafeon might not break any records when it comes to power, but chances are you'll want one just because it's cute and it's an Eeveelution.
Eevee can also evolve into the Ice type Glaceon in gen four. This one might see more use than its partner Leafeon just because good Ice types are hard to come by. It's also worth mentioning that Eevee will be featured in August's Community Day event. This means that Eevees you capture will have an increased chance of being shiny. Shiny Pokémon tend to be a trophy among collectors, so even if you're not an Eevee fan you might want to nab a shiny Eevee and evolve it into a shiny Glaceon just for trade purposes.
As its Psychic and Fighting type combination suggests, Gallade packs a punch. Ralts that are male become this swift fighter that will be great for raids and gym battles. As a bonus, it also gains access to Mega Evolution in gen six, making it even more powerful. Mega Evolution probably won't hit Pokémon Go for at least another year and a half at the rate the game is going, but it's always good to plan ahead.
Magmar gives Blastoise some competition in the cannon game when it evolves into Magmortar. Critics love to attack its design, but Magmortar's arm cannons give it access to an astounding variety of attacks. Electric type moves and even the powerful Solar Beam are at its disposal, at least in the main Pokémon games. Who knows what Go will let it have? Either way, it's still worth your effort to stock up on Magmar candies as its evolution is a solid jack-of-all-trades.
Piloswine is one of those Pokémon that shows up in raid battles in a pinch. It knows some fast Ice type moves and can tank fairly well. There are certainly better options, but there are also many worse ones. In generation four, however, Piloswine gets a huge power boost by evolving into Mamoswine. It'll become a top-tier tank and probably have strong attacks to boot. It will also likely cost a lot of candies to evolve, so you better start catching those Swinub now.
That's only a small sampling, the few gen four evolutions that will likely make waves (despite none being water types). However, you'd also do well to pay attention to Togekiss, Dusknoir, and Gliscor. Overall, gen four's addition will definitely make for an exciting time to be a Pokémon Go player.