When do you think gen 4 will be added?

Half in September and half in November. That’s my guess.

6 months seems to be the gap. Spring and fall.

I reckon there will be a month of each Regi titan, then we'll be introduced to gen 4 through Regigigas (if other mons haven't been trickled into the game first, like we had with gen 3)

I'd say sometime in the Fall is a safe bet. However, if we want to put our tin foil hats on, the datamined Celebi special research dialogue text from Prof. Willow for Go Fest makes multiple references to needing Celebi's help to fix spatial distortions that are affecting Pokemon's habitats at the event that are caused by some unknown force. Spatial distortions are kinda Palkia's thing, so perhaps Palkia gets unveiled as the first Gen 4 raid boss at Fest? It's probably very unlikely, but it is still in the realm of possibility.

Link to the dialogue text for the Go Fest special research:
https://www.reddit.com/r/TheSilphRoad/comments/8v5k5j/new_gofest_quests_celebi_as_special_research/

So far it's been 10 months following the last gen introduction. So it should be in September. These Regi raids are gonna be a long drag. Trickling in by late August I hope.
More than anything I want the damn gym battle system updated.