EA has announced that those that own and registered a copy of The Sims 3 on Origin will automatically receive in-game content in The Sims 4. In addition, those that purchased expasion packs will also be offered with additional rewards.
According to EA, a total of 13 in-game rewards will be available based on the content you own, one reward for owning the game, one for each expansion purchased, and one for owning everything. Although it's was not specifically revealed what these rewards do, EA has stated that each reward "will affect your Sims' emotions and personalities in a unique way."
The Sims 4 will be available on the 2nd of September in North America, and on the 5th of September in Europe.
Source: The Sims