The Sims 4 opens in a resolution bigger than my screen.

Try starting the game in Windowed mode to see if you can change the settings in your options then: http://answers.ea.com/t5/The-Sims-4/How-to-start-The-Sims-4-in-Windowed-mode/m-p/3447611#U3447611

If that fails, try this

1. Close the game
2. Open the file documents\electronicarts\thesims4\options(.ini) with a text editor
3. Search for an entry with resolution in it (not sure of the exact name)
4. Change the two numbers to match your screen resolution (the third is the refresh rate, you can leave that)
5. Save the file
6. Open the game.