I can't think of any gameplay issues in either game that are significantly fixed in FO4.
There are a lot of incremental improvements. In the context of the original question, FO4 is a
lot like FO3 and NV, so it's difficult to see that anyone who didn't like those two is going to be swayed by those changes. But there are still a lot of differences.
Off the top of my head, settlements, weapon mods, crafting in general, critical hits, the perk-focused character advancement, speech checks, power armor, explosives, VATS is no longer completely paused, healing takes time.
Not all of which are improvements, dialog's significantly worse than either prior game. The console-focused D-pad interface is very limiting, since there are always EXACTLY 4 responses to everything, and we can't see what the hell we're going to say before we say it. Die roll based speech challenges are fraught with problems, NV handled speech checks in a much more intelligent manner than FO3, and Bethesda decided to ignore that.
Still, in most areas the changes are a significant boost over FO3. Weapon variety is better, we aren't all toting around Chinese Assault Rifles that only really vary based on how repaired they are. Companions are far, far more interesting than FO3, roughly on par with NV's companions (even if none has quite as complicated a quest line as Boone). I like how often I stumble across firefights between different factions, which was a relatively unusual (if not unheard of) occurrence in earlier games. Player-crafted food healing items are now usually my primary source of healing, rather than a poor alternative to stimpacks. Grenades are a practical combat tool rather than suicide devices.
Not everyone likes the settlements, but I do. I just like building and crafting things, and there's room for considerably creativity there.
