I disagree with Marley being a star of the Season. I mean she's had a lot of songs and airtime, but her character hasn't really developed much IMHO. I mean she's shy, and when she tried to be confident in asking Jake to Sadie Hawkins, she lost her composure. I also feel that her line in the Diva episode to Tina just before they sang 'Diva' was completely Out Of Character.
As much as I love Blaine, he hasn't really developed either, apart from learning with live without Kurt (by his own fault) and leading New Directions (with Sam, I guess). I think he is an amazing character (with a beautiful portrayer, but that's a different story) but besides his breathtaking solos, his character has been quite two dimensional this Season.
Now Sam I have enjoyed watching this season. He has been funny, emotional and romantic. He's has some good numbers (Copacabana was entertaining, as was Wake Me Up Before You Go-Go) like Heroes and watching his relationship with Brittany almost reminded me of when I loved him the most; when he was with Quinn. I think that he's maybe got that back a little and I love it.
Kurt is my favourite character BY FAR in this show. I just watched Laryngitis which never fails to give me chills when he sings Rose's Turn, but it just showed me why I love him. In Season 4, there's been glimpses of it, but I find myself craving Season 1 and 2 Kurt again. He's had nowhere near enough screen time and nowhere near as many songs as I'd have liked, but somehow he's managed to capture my interest again. I go into every Season and Episode with an open mind, and Kurt's little gestures and lines make me love him more. For me, Being Alive was breathtaking and I love his relationships with Rachel, Santana and Isabelle. He's truly grown as a person in the short time he's had this season and his persistence to get into NYADA proves this. The Klaine breakup killed my heart, but I think it was best for Kurt as a person. I think it's time for them to reunite. They've grown as people apart and together this time, they'll be stronger than ever.
Honestly? The real stars for me have been Ryder, Santana and Kitty. Let me explain.
Ryder: With the help of Finn, he has progressed with his education and even managed to become a better friend. In I Do, I was very impressed by his acts of friendship to Jake, with helping with his Valentine's gifts/surprises for Marley. Fair enough, he kissed her, but that was because of his feelings for her and his frustration that he had organised everything and still she chose Jake. Though I disagreed with his comments towards Unique in Feud, I love Ryder as a character and have enjoyed watching him and his character development.
Kitty: Kitty has changed a lot since The New Rachel. She's developed feelings. It was the lack of feelings that set her apart from being like Quinn, because whilst Quinn was harsh in Season 1, she still had emotions. Kitty had been fun to watch, and I thoroughly enjoyed her rendition of Look At Me, I'm Sandra Dee. Even though people complain that she's just like Quinn, she's a completely different person in some ways. She has proven that she's meaner (just look at the Marley/Kitty scenes in every episode) but she has also proven that she is kind. The way she defended Unique when he told the story of being followed home was admirable. I think she's been great to watch.
Santana: Santana Lopez. Season 1: A Mean Bitch. Season 2: A Mean Bitch, but developed emotion around episode 15. Season 3: More sentimental, but still the sassy princess we all love. But Season 4? She has really come out of her shell. She took charge of her and Brittany's relationship by breaking it off. This helped them both as they wouldn't feel guilty if they felt something for anyone else. When she fought for her in Diva, I loved it. She showed that she still cared. Arriving in New York was the best thing to happen to her. She learned friendship. True friendship with Rachel and Kurt and I hope those three form a solid trio. I really like them being best friends. She protected Rachel (tried) from Brody as she rightly believed that he was going to end up hurting her. Even though, she still likes to stir the pot (Kurt/Adam and Moulin Rouge-gate), her intentions have become cleaner and I just think that she's been the overall delight of the Season.
Sorry this has been so long, I just needed to state my views in one long rant. :)