If you saw couch's post it sounds like there is a 5th team with a small number of people also working on something yet unannounced.
I have always discounted it though because Brandon Adler is the game director of TOW2. Given the size of the TOW2 team it is very unlikely he is managing another project and I have seen nothing to back up that so far.
It also says Carrie Patel is the game director of TOW2 which is incorrect. Carrie is the game director of Avowed. Given Carrie did a significant amount of the writing for Eora/PoE2 this always sounded suspect to me.
I believe Chris Parker is not running any games at the moment. I imagine the role of COO is now significantly more time consuming than it used to be given the size of Obsidian now.
Adam Brennecke is running Grounded which only leaves Brian Heins to be leading something else. Brian has done Tyranny and that is it. I somehow doubt they are giving him another new project