Although the foundation of the Sims 4 is flawed when it comes to deep long term game play and unique personalities, there is an area of game play that Sims 4 (in my opinion) does a much better job at executing compared to previous games, and ultimately is the main reason why I prefer Sims 4 over previous titles.
I find that Sims 4 offers many different tools and game play features that make a sims external lifestyle extremely deep, detailed and unique. No two households feel the same in Sims 4 to me, even if the sims themselves may behave very similarly.
Lifestyle is the key word here. Which world my sim lives in, the varying game play opportunities in each world, what lot traits are applied to the lot, what career they pursue, what skills my sim has, what clubs my sim participates in, what NAP's are in place, what the eco footprint at the time may be, etc.
In Sims 4, I can create a sim that lives completely off the grid, no power sources whatsoever, this sim collects their own food through gardening and fishing, they make a living by completing odd jobs and selling collected/crafted goods by using a sales table. They can join a club dedicated to rejecting electronics. Using NAP's the neighborhood they live in can further deepen their lifestyle, and as a result of their simple low maintenance life, nature thrives. I can make a holiday dedicated to celebrating the simple things in life. - I can also create a sim whose lifestyle is the complete opposite, they live in a normal house outfitted with all sorts of electronic entertainment. They can pursue a variety of electronic related careers and skills. They can use alternative energy sources to lower their power bill because of how often they use electronics. They can be in a club dedicated to playing video games. NAP's and lot traits can be set in place to further deepen their lifestyle, etc.
I can create an extremely wicked evil sim. They live a life of dirt and crime. Using lot traits, NAP's, and the eco footprint, I can quite literally make a "bad" neighborhood, one which sims fight each other, wear bags and constantly cough because of the bad environment, where the lot traits promote anger, mean behavior and dirty surroundings. This sim can join a club dedicated to ruining other sims lives. I can create a holiday dedicated to being mean to everyone else. This sim can pursue careers and skills that expand their mischievous life - Again, I can also create a sim whose life is the complete opposite, where their entire life motto is living a life of good virtue.
These two examples can also have variations, the exact same sim (in either or both examples) would have different experiences living in a City compared to living in a desert, or any other world for that matter. Each world usually has some unique game play (San Myshuno with festivals, Sulani with the Island, etc.), each world experiences seasons differently too. I find Sims 4 to have the most variation in careers as well, the same career can be experienced differently if you decide to play with/without fame or with/without post secondary education, on top of the traditional careers there are also work from home careers, freelancer careers, odd jobs, businesses, self employed, and a variety of other ways to make a living outside of just careers.
In conclusion, there are many tools and features in Sims 4 that flesh out my sims lives in extreme detail and depth, in ways that Sims 2 and 3 just don't deliver. I find Sims 4 expansions expand lifestyle opportunities, while Sims 2 and 3 expansions expanded the core lifestyle that's been established in the base game. These are just my thoughts on Sims 4, giving credit where I think it deserves credit, It's just a shame that the sims themselves are flawed and shallow compared to the external life around them. If the developers ever manage to fix the shallow sims, where the sims themselves react appropriately to their lifestyle depending on their personality, the entire game would simply be better, everything would have that much more depth to it, and maybe at that point arguably deserve the title of best Sims game in the series.