The Sims 4 Create-a-Sim demo is available for free on Electronic Arts' Origin site. On Origin.com, if you search for "Sims 4," you are taken to the game's page.
If you don't have an Origin account, click "Register" on the bar at top of the page and create your account using your email address. After logging in, click "Download Origin" from the same navigation bar if you don't already have the program installed. Then, click on "Try The Sims™ 4 Create-a-Sim Demo today!" above the T for Teen ESRB rating. This will tie it to your account. Open Origin and you can install it from there for free. The file is just over 1GB and gives you access to the Sim creator only. If you enjoy the demo, the full game is available for purchase digitally from the same page.