Sims 3 will not be compatible with Sims 4 unless EA decides to completely change how things have always worked in the past.
When the Sims 4 arrives, what will happen to all of the Sims 3 worlds and games? Will they be obsolete or will they merge with Sims 4? In order words, will I have to start all over purchasing Sims 4 worlds?
Sims 3 content will most likely not be compatible with Sims 4 (it wasn't for any of the games in the series).
This does not mean that Sims 3 will be obsolete though. You can still play it, use the worlds in Sims 3. There just won't be new stuff for it (they did say that the Store will continue for some time but not how long). You don't have to buy Sims 4. There are still many people enjoying Sims 2 so, it's not like the content is lost.
Sims 4 will just be a different game with the same topic.
