dreap1 Your expansion packs will be deleted along with the base game, but they'll be available to reinstall: your account owns the packs, not your computer. You don't need to delete anything manually—just tell the EA App to uninstall the game and it will take care of the rest.
Your saves shouldn't be deleted when you uninstall Sims 4. Saves and other user data are stored in an entirely different location: Documents > Electronic Arts > The Sims 4. The game itself is likely in Applications > EA Games or Users > your username > Applications, although you could have selected a different location when you originally installed the game. Point is, the game itself gets removed but the user folder does not.
However, I'd still strongly suggest making a backup of your saves. This is less about the uninstall process than about having an extra copy should something else delete or corrupt your saves. You can simply copy the saves folder (inside Documents > Electronic Arts > The Sims 4) to another location, preferably an external drive or USB, but if you don't have one handy, a different folder on your Mac would be okay as well.