If a gen 4 wave is coming....

Some of them have special evolution or quirks that make them unable to be released. Smeargle still hasnt been released. Kecleon is somehwat like a Ditto. Spinda has multiple forms. Nincanda has a special evolution that would be weird to implement.
I do see Clamperl's evolution line being released also if the evolution event is happening.

Keep in mind, not all of gen 2 has been released either, smeargle and celebi as of now arnt out

There are rumours that say that Smeargle will replace Mr. Mime because Mime Jr. is going to be released, which is a baby Pokemon that everyone will be able to obtain from eggs. There are also speculations that the next even is going to be an "Evolution Event", and in that event all of the evolutions from the previous generations are going to be released. Meaning that at first, you will be able *only* to evolve the already existing Pokemon into their Gen 4 form. So if it will be the case, the only first Gen 4 release will not really release new Pokemon, just the new evolutions. I think that Niantic would release Clamperl and it's evolutions with that but the other ones are problematic.