March 2020 - last edited December
Unfortunately EA does not make buying the Sims 4 very easy for Mac users due to Origin trying to persuade you to subscribe to EA Play which is Windows only. Mac users cannot download the trial version of the game either.
Do not subscribe to EA Play or EA Play Pro
Go to the Origin website or open the Origin app, navigate to the game and click Get The Game:
Ignore all the adverts to subscribe to EA Play, it will not work on your Mac. You need to buy the game outright:
Choose which version of the game you would like to buy and follow the instructions. If you see any other prompts to subscribe to EA Play ignore them.
If you have subscribed to EA Play you can cancel your subscription by logging into your EA account here - https://myaccount.ea.com/cp-ui/aboutme/index then click on the EA Play tab and follow the instructions to unsubscribe. You can also access the account page by hovering over your user profile in Origin and clicking EA Account & Billing.
If you have inadvertently 'upgraded' your previously purchased game in Origin to work withEA Play you need to revert it back. Right click on the game in your Origin games library and select Uninstall then choose Revert game edition: