SimObsessions There should be a check box in the graphics settings, to the right of post processing. If you don't see it, make sure you haven't forced DX9 mode within the EA App. Open your game library, click Sims 4 > Manage > View properties, and if -dx9 is in the command line box, delete it. Then the game should automatically use DX11 mode, and you should see the option to use DX9 mode in the graphcis settings.
You can always see which mode you're using in the lower-right corner of the Main Menu, to the right of the game version number.