The information in this post is no longer current. There’s no registration needed you can just download the CAS-Demo through Origin.
You can now register for the Sims 4 CAS Demo.
Be aware that this is still no guarantee you’ll get it but at least people that don’t want it, won’t get it this way.
If your country is not listed, choose another that has a language you understand.
To register follow these steps (Source: EA)