Honestly, I don't need sign something that I know for sure is already in development. The producer Graham Nardone has said a few years ago, in ModTheSims forum, that The Sims 3 Pets Expansion Pack was in development since the base game was released in 2009. The point is that an expansion pack about animals is always a of which take longer to develop, and I do not need to explain why, right? Let's take it easy! The Sims 3 Pets was released two and a half years after the base game release, because its very complex.
The Sims Unleashed is the 5th expansion for The Sims 1.
The Sims 2 Pets is the 4th expansion of The Sims 2.
The Sims 3 Pets is the 5th expansion for The Sims 3.
I do not think it will be different in The Sims 4. The issue is that we are just talking about an expansion that takes longer to be developed in The Sims.