While it is not a piece of custom content or even limited to Sims4, I want to link to
Sims Community Social. Although sims forums are not a new concept I feel like a good chunk of the sims community has kind of moved away from them and I miss how involved we all used to be. Tumblr is a great platform and it makes it really easy to share things, but it kind of lacks the close-knit community feel. I hope the recent launch of SCS will help bring us back to that.
And of course it's not just a forum. There are resources (downloads) and a media center.