Uninstalling The Sims 4 on a Mac is really easy because of Origin.
Firstly open Origin and click on Games Library. Then navigate to The Sims 4, right click (or hold down CTRL and click) on the game and select Uninstall:
You’ll get a warning window asking if you’re sure you want to uninstall everything, just click Uninstall:
Next, navigate to where your game was installed to on your Mac. Typically this is your Applications folder in your Mac user account. If there is a folder there called The Sims 4 Packs, right click on it and select Move To Bin. This is the folder that contains all your DLC. Make sure to empty your trash as this folder will take up quite a chunk of storage depending on how many packs you have installed.
Uninstalling shouldn’t get rid of all your user data (saves, screenshots, mods etc.) but backing it up is always a good idea. You can find the folder by navigating to Documents > Electronic Arts > The Sims 4.
If you’re having problems with the game and Origin then you may need to completely uninstall Origin too. This is a bit more complicated as you have to search for the files to delete manually. You can find full instructions on cleanly uninstalling Origin here.
If your game isn’t behaving properly it’s always worth trying to repair the game before uninstalling it. You can select Repair Game instead of Uninstall from the drop down menu when you right click on the game.
And finally, if your game is not even uninstalling properly, your best bet is to uninstall Origin and start again.