EA/Maxis currently have no plans to do a Sims 4 store. I would have to dig up the announcement, but they pretty much plan to release everything through their online platform, Origin.
As I become aware of each new piece of new content, I will list it in this thread:
The Sims 4 Downloadable ContentIf EA/Maxis decides to make a separate store, I will post that as well, but as I mentioned above, I doubt this will happen.
However, the Sims 3 Store was actually up and running around the time when the base Sims 3 was released. In fact, the first release was the free world of Riverview. Origin actually came afterward when EA decided to run their own online digital distribution platform rather than go through Steam.