Scaling for items in the Sims 3 was... odd. A dining chair is usually around a foot and a half wide in real life. An overstuffed chair can be three feet or more. Yet, in the Sims 3, they both took up the same space (1 square). I usually assumed a six foot bookcase was as wide as a single square window in real life. (As that's how they work in my home.) So, I used to just divide by 2.
As for a 12X12 sizing down to 4X4? Well, that worked for some rooms, but it didn't work for my Sims 3 bedrooms. In that space, you could fit a double bed. In real life, you could fit a double bed, bookcase, dresser, and a few other things if you felt like crowding.
I will be getting the Sims 4 this weekend (I hope) and I'll let you know what I decide on for scale. It greatly depends on how the pathing works. (i.e. Can sims take advantage of the half tile area where a piece of furniture isn't?)