The newly announced Sims 4 Rewards program will give you one in-game item per Sims 3 expansion you own, as well as one for the base game itself. Each of these items is a different-colored lamp in the shape of the Sims logo that will "affect your Sims' emotions and personalities in a unique way."
There were 11 expansion packs released for The Sims 3, meaning there are 12 lamps to collect in all. Should you be enough of a fanatic to own the full dozen--I, myself, will have to make do with two measly lamps--you'll get a bonus reward in the form of the "Ultimate Freezer Bunny award."
To receive your rewards, you need to have either purchased the game or expansion through Origin, or registered it through the official website. Once you've done so, visit the rewards page and the corresponding items will unlock so that they are available in the Build Mode catalog in The Sims 4. You have until December 31, 2015 to claim your rewards.
Sims' emotions are a point of emphasis in The Sims 4. The focus on this aspect of the game is one of the reasons cited for the lack of toddlers and pools, which have been a major point of criticism among fans. Further fueling complaints from fans is the prospect of a Battlefield Premium-esque service that EA may plan on offering.
The Sims 4 is scheduled for release on September 2. EA recently announced its system requirements, which you can view here.