It looks like more Pokemon from Gen 4 are soon to be added to Pokemon Go. Bonsly, Croagunk, Tangrowth, and Yanmega are the next Pokemon from the Sinnoh region to be added.
Generation 4 Release
The 4th generation of Pokemon started to appear in Pokemon Go from October 2018. Similar to previous generations, gen 4 Pokemon are being released in stages.
For instance, the first batch of generation 4 Pokemon released 26 variants. These included the Sinnoh region's starting Pokemon, Turtwig, Chimchar and Piplup. These were also part of the December Community Day.
Pokemon Go Leak
Three batches later and Pokemon Go fans are wondering which Pokemon will be the next to be released. Well respected Pokemon Go dataminer Chrales appears to have the answer.
The datamine suggests it will be just four new Pokemon to next appear in the mobile game. No other generation 4 sprites were found during the datamine.
— Chrales (@Chrales) December 6, 2018
Next to be Released
The four Pokemon set to be released offer a good variety.
Bonsly is the baby version of Sudowoodo. Croagunk was an entirely new Pokemon added to gen 4. Finally, Tangrowth and Yanmega are further evolutions of Tangela and Yanma respectively.
The Sinnoh region was first seen the Pokemon games: Diamond, Pearl and Platinum. This was the first generation of Pokemon introduced that included Pokemon related to those of previous gens.
Generation 4 introduced 107 new Pokemon in total. Consequently, there are still 56 Pokemon to be released, assuming it is just the above four that are next to appear.
Pokemon Go PvP
Whether this upcoming release is part of the hugely anticipated Pokemon Go trainer battles is currently unknown. There has been a lot of promotion and work put into the PvP battles. Consequently, this suggests this mode will be next. Furthermore, the official Twitter has already stated that this mode is coming this month!