Where's the 4th generation Nest Learning Thermostat?

Curious if anyone has any idea when we’ll see an update to the Nest Learning Thermostat? I want to purchase one for my home, but don’t want something that’s going to be obsolete by the time I have it installed.

FYI, I’m not talking about the newer, cheaper model. When will we see an update to the Learning model? The current third-generation model that has been out since before Google purchased Nest.