Yes. There will be two categories - the Sims 4 on top, Sims 3 on bottom. We will even keep the Sims Medieval archive. If you were around during that time period, you'd know how it'll work. The categories are collapsible so if a person didn't want to see a list of boards for TS3 on the main page, they could collapse that so that only TS4 will be visible. This will make it functional for people who are still playing Sims 3, while allowing for new discussions to take place within the Sims 4 category and its boards. It'll take some discussion with staff, but it's likely we won't create boards for TS4 until the day the game launches so that we don't have a lot of threads with speculation to prune when we have the real deal in our hands on launch day
If you look at the top of the board index, you will see on the green bar where it says "The Sims 3 Forum". That is the Sims 3 category. Far to the right of that is a '-' sign, which can be used to collapse the category. So, we'll just have another category when TS4 hits. Players who don't own TS4 can collapse that and use it as a Sims 3 Forum. The address will be carls-sims-4-guide, as you might have guessed.