There are a number of free trophies for your Sims 4 game for Sims 3 players that you can place in your house as decoration
The Sims 4 rewards are plumbob statues, that are unlock automatically in your game if you have Sims 3 games registered to your Origin account.
You need to have your Sims 3 games registered to the same Origin Account you’re registering the Sims 4 to. The Sims 4 will then know which Sims 3 games you have and will automatically unlock the content in your game.
There’s nothing else you need to do then make sure you have your Sims 3 games registered.
You can find out which rewards you’ll get today. Go to this site: The Sims 4 Rewards and click on the button on the bottom of the page.
to see what you qualify for (you might need to login to your Origin account)
There are 13 reward statues: 1 for the Sims 3 base game, 11 for each add-on and 1 if you own everything. There are no rewards for stuff packs.