I've been playing the Sims since Sims 1. I have bought and purchased every expansion and sequel and stuff pack since then. Obviously they want to continue making money and will release expansion packs for Sims 4. So yes, if you owned everything for Sims 3, none of that stuff will be in Sims 4. I'm sure it will come down the road as an expansion but better, hopefully. The base game of Sims 4 however so far seems slightly improved from Sims 3. Like KoolAid said, Sims can now multitask so you can play a game using your tablet while watching TV to fulfill your Fun need even faster. Another example is my Sim was eating and I told him to go change outfits. Instead of stop eating, he carried his plate of food with him, changed in the bedroom and then finished eating.
The new town (there are two technically) does seem smaller in comparison to Sims 3. The building mode is way better than ever before. Buy mode and build is now just combined into one mode so you can do everything from there without having to switch. For someone like me who doesn't really get into building but needs room to add more stuff I buy it makes it easy. For example, I wanted to get a desk and computer in the house. I just went to build mode and under the Study section found a furnished room with what I wanted, plopped it down on my house and it added the room right to the rest of the house. Too easy.