In my opinion, they were all bad except for sleepless in Ponyville, But that's my opinion.
Okay, here is a list that made Season 4 absolutely great and a season I'd watch again and again (and already have)
The rainbow powers arc was excellently executed and was a very good way to handle advertising a new range of toys, last time they did that it was a canterlot weddings Candance and Shining armour and before the episode aired people raged hard over another Alicorn out of no-where but it simmered
The premier and finale were both excellent in my opinion, yes the finale had some flaws but nearly all of the finales do. This one I could watch again and again though.
The return of Derpy after her awfully long absence from season 3
Dave Polsky entered the writing team and did a excellent job too, entering with Castle-mania one of my favourite episodes so far of the series
Maud Pie
rocks
rocks
and more rocks
Speaking of extra characters we saw a array of new characters that were likable and enjoyable to watch like Seabreeze, CoCo, Silver Shill, Cheese and so on
WE TRAVELLED THE ROAD OF GENERATIONS, JOINED BY A COMMON BOND!
I'll wait here for you to finish the song
The re-introduction to an old villain, that being Tirek, this shows that the show can easily adapt old generation villains and opens up so many doors for world building.
List of episodes I'd watch again and again (and already have) : 1, 2, 3, 5, 6, 8, 9, 12, 14, 18. 21. 23. 25, 26.
Return of Flutterguy in an amazing episode.
Testing Testing 123 is also an excellent episode and speaks to me on a personal level because I had to learn differently from the norm.
I could go on but the list would go on for ages, long story short Season 4 is my favourite season