You can see the trends
here. If I were to guess, I currently expect Update 4 to release in early June, which would be a little over 5 months from Update 3. I think there's going to be a lot to cover (with this being the big Exhibition showcase), and we know even the pre-Exhibition material was considerable. So this is probably going to be an enormous update that will take a while to finish, but should still be well worth it for the end result.
Hopefully the development time will drop back to the 3-4 month range when we begin Chapter 4, but we can chart that bridge when we come to it. For now, we can just wait patiently. And chart any numbers TD gives us!