So much repetition in this thread. Here's mine: Sims 4 allows for more genuine creativity. When presented with every possible tool imaginable, like the color wheel and constrainfloorelevation, it's extremely easy to make ANYTHING, and to get lost in building something exactly as you imagined it. In Sims 4, there's much more room for creative work-arounds. No platforms/friezes? Enlarge the fountain slab from Spa Day. No ladders? Use shelves. No bunkbeds? Raise up a bed and use pillars to make it look connected. No color wheel? You'll have to work harder at making furniture match, many times creating a color scheme you wouldn't have thought of in Sims 3.
I'm a builder exclusively so I don't care if my builds are playable, but a lot of the time they are. Sims can use beds that are raised up. They can use hot tubs that are sunken into a foundation. Sims 4 challenges me to find new ways of making real-world features without simply having it handed to me if that makes sense.