The Redditor that goes by the name flakride submitted a post on TSR, explaining why we should expect Gen 4 sooner than we think.
Here is what he says:
“Gen 1 was released on 5th of July 2016 and Gen 2 was released on 16th of February 2017, but without the baby mons. The first wave of Gen 3 was released on 8th of December 2017. The last wave of Gen 3 was released on 9th of February 2018.
The release period between Gen 1 and Gen 2 was approximately 8 months and between Gen 2 and Gen 3 was around 9 months. The time between the first and the last wave of Gen 3 was around 2 months. Gen 3 was a lot larger than Gen 2, so we were waiting a bit longer. Between Hoenn and Johto many new things were released. This includes the gym reworks, the new raid system, the weather boost and many other things.
The bigger part of Gen 4 Pokemon are the baby/evolver versions of the Pokemon that already exist. We should consider that the Sinnoh dex is not much larger than the Johto dex.”
The Reddit user thinks that Niantic will stagger the Gen 4 release just like they did with Gen 3. He suspects that we could see the release of Gen 4 as soon as June, or maybe even May.