Jesus, I'm avoiding spoilers like the plague here, but based on the 26 minus final 3 episodes that I HAVE seen, I can't say that this fourth season has done much for my waning interest in MLP. Certainly some individual elements of individual episodes were extremely well-done, as was the comedy and music for the most part, but it seems to me that the writers have just consistently gotten the characters WRONG. Too much Flanderization, not enough nuance. Pinkie Pie, my personal favorite of the Mane 6, was reduced to little more than obnoxious one-line screamer. The one-liners weren't even funny, just... loud. Because that's all they knew to do with her. It made the episodes that focused on her so very, very hard to watch.
Brash Rainbow Dash was brash, Overbearing Applejack was overbearing (though to a lesser degree), Saccharine Fluttershy was saccharine, etc. etc. They didn't even do anything with Twilight's supposed newfound powers/status/wings. She didn't change, and while some of you might say, "Well, that's a good thing, right?" I'm of the opinion that if she didn't need to change, why change her? As far as I'm aware, Alicorn!Twilight isn't doing much to increase MLP merchandise sales, so there's not even a marketing reason for her transformation. (Sorry if you saw my previous post in whatever thread ago that basically reiterates the same point, but it's something that still irks me.)
The moral lessons espoused lacked the same... not necessarily depth, but close to it... as the first two seasons, where the characters were at their strongest. Part of the problem, I think, is that there's just not much more they can do with the Mane 6. Lessons have been learned, days have been saved, friendships have been forged. The writers don't want to spread any of the stories over multiple episodes, so they're left repeating the same concepts as before. Sometimes they manage to put a nice spin on it, more often not.
I want to see more Spike. Completely random thought, but I would like to see more episodes centered on him. Not morality tales, or Spike-as-incompetent-joke-character, but just him being him. He's got a lot more going for him than most of the ponies do by nature of being so different from them yet coexisting with them. And I understand how that seems like a recipe for his Dragon Quest episode or whatever it was called, but there's no reason they can't play it up a bit more. But maybe I just like Spike and dislike the way they've turned him into comical relief.
The expanded mythology doesn't fit perfectly with what's come before. Just like how Cadence and Twilight break down the omnipotence associated with alicorn-dom, so too do the breezies and fruit bats and other new creatures clash with the established canon. Or maybe it's not so much their sheer existence as it is how they were treated by the writing. They should be common enough in their world that the ponies take it in stride, but sure enough, every episode has to have a character espousing exposition to another character who is not an audience surrogate. Fluttershy explaining the breezies to her friends should only happen at their prompting, not because she feels the episode's prologue timer counting down. This happens far too often in the fourth season: exposition doesn't unfold naturally through story progression and character interaction, but through forced get-it-to-the-audience exposition dumps. Part of this has to be due to episodic storytelling and time constraints, but if they could manage well-written episodes in the first two seasons, why couldn't they do the same afterward? (Well, the answer to that has to do with Lauren Faust's departure from the project, but more broadly, I'd say it's because the writers just aren't very interested in these characters anymore.)
But that's not to say season 4 doesn't have a lot of good things, either. Certainly it's a major recovery from the stumble that was season 3 (and the finale of season 2), and there are a ton of really good comedy moments - Pinkie Pie's quiet pan-up/pan-down scheming smile moment from the barbershop quintet episode was hilarious, as was her gloating afterward, and her egging on Fluttershy is likewise top notch (that episode in general was really top notch). I think things are just hampered a bit by the marketing priorities pushed on the show by the suits; when it was less about that and more about character stories, the series was at its best AND did a better job marketing its wares. (No one really thought about Twilight's balloon as a marketing ploy, but everybody saw Cadence and Alicorn!Twilight for what they were.)
Anyway. That's my two cents.