There are some places where TS4 is getting better. They finally added Toddlers, and they're actually much more fun than in TS2 as playable sims. They have fixed a lot of bugs...even though new ones have arisen to take their place. However, there are also still a lot of flaws...a lot.
1. The game mechanics introduced in the "Parenthood" pack that help flesh out the Child and Teen stages really should have been patched into the base game. It's stuff that should have existed in the base game, and (like hot tubs and diving platforms) should not have been something we had to buy as an add-on...particularly because the Child and Teen stages have so little exclusive gameplay in the base game.
2. We still have barely any gameplay options without mods. We have a four-speed aging system (Off, Short, Normal, or Long) that we can't customize, we have no means of controlling townie generation in the instance of normal sims or supernatural sims, can't manually manipulate alien pregnancy and abduction rates in the options menu, no story progression tuning settings (some of us want all of the important stuff in our sims' lives to be done by our hand, some want to concentrate on just one sim while the others are played by the game's AI, some want something in between...no one is getting what they want in terms of story progression with the current default settings)...and the list of missing options goes on.
3. The massacre of the memories system. At launch, it was a cool spin on the old mechanic. We took screenshots of our sims and could pick whether or not they became memories, we could assign whatever emotions we wanted to them to induce those emotions in our sims by having them recall them, and when we went into the memories menu, we only saw memories from our current save. Then, they swapped the memories system for that jacked-up, laggy "Screenshot Manager", took away our ability to manually assign emotions to memories (which was a huge blow to gameplay, particularly for those of us who liked having a way of having our sims' negative emotions punch through that miasma of Happy) and instead had the game auto-assign emotions to memories with no way to hand edit the emotions at all, and made it so that when we bring up the menu, we get every screenshot in the whole folder from every single one of our saves in a hard-to-sift-through mess! I used to use the memories system all the time when the game first came out. After the "Screenshot Manager" upgrade, I never touch it anymore...and it was one of my favorite new features in TS4, too!!
4. The generally horribly unbalanced state of the emotions system. The game is totally overloaded with ambient Happy moodlets which totally bury almost any negative moodlet, and negative emotions have no booster emotion the way that positive ones have Happy. It's a hot mess, no two ways about it.
5. Relationship decay for inactive played households. It should never have been added without a way to turn that ish off...especially in a game that was marketed as bringing back TS2-style rotational play. No, Boo. You know you wrong. *pursed-lipped expression of sassy Black girl disappointment*
And these are just things that have yet to be fixed in the base game in terms of Live Mode gameplay. I haven't even gone into how the sims' AI seems to have become dumber over time or any of the Build Mode complaints. There have been things added to the game that have made it much better in some areas, but in other areas it has remained in stasis, improved only slightly, or gotten worse. And the hardware limitations aren't even a valid excuse, due to TS2's base game having been so much stronger, but having been made to run on much wimpier hardware in terms of recommended specs than TS4 is made to run on minimum specs. It's just poor behind-the-scenes organization from nose to tail on this one.