Ok, so I have a lot to say about this. Here goes: 
 This has been on my TBR list, like, since I've joined. I only realised there was a fourth book in the series when I joined Goodreads. At first, I was a bit hesitant how good it would be and how the author could still keep it interesting. There's now been a lot of hype-ish/attention, both good and bad, but in the end I succumbed to reading it, wanting to form my own opinion on the story, and also having enjoyed the Selection series so far. 
 As a side note, this book is not necessary for the story. I know I could have just read the three books and then left it there; Eadlyn and co. weren't at all necessary to the series. But now that I've read it, contrary to many other reviewers, it seems, I actually quite enjoyed it and don't regret it at all. 
 I gave this book 4 stars; I gave the other books in the Selection series all 3 stars. I am quite surprised that I liked this book more than the previous ones but... I did. So the good things I liked about this book were: 
 a) The cover is divine; definitely the best one out of all the other covers in the series! I liked the colour scheme and the dress, they were both really pretty. 
 b) I liked the name Eadlyn, though I'm still not sure how to pronounce it. I think the name is pretty and America's name was a bit far-fetched. 
 c) (good/bad) At times, especially at the start, I have to say Eadlyn's behaviour and attitude was selfish or bratty; she said stuff like a really bossy person would say, even the Queen wouldn't say it like that. However, I liked that she was very decisive, focused and responsible with her job and knew what she wanted and how her life would pan out (until, of course, she didn't, and realised having a husband/prince/boy/friend isn't so bad, though she isn't entirely convinced until the end, and even then she isn't eager or anything, which still stays true to her character). Through the book, Eadlyn interacts and becomes friends with some of the boys, and this really helps her to mature and grow overall. It was a gradual thing and now she is a bit unsure of herself and is not yet Queen material. So, in essence, I think she does get a better personality and is a better person, but not overly so. 
 d) Since Eadlyn is the Selector, we get a different perspective from previously (when America was one of the Selected). It was interesting to read from someone who was born into and grown up as royalty and has to choose from 35 guys. I think I liked this perspective a bit more than America's; it was more interesting-ish. 
 e) Because of the above, there are a lot more guys. With America's case, she only had to choose between Aspen and Maxon (and we all knew how that would turn out, at least I did, I mean if she was there but was never gonna be with Maxon, the whole thing would be kind of pointless, but anyway), so again it was different. I think I liked more no. of guys and though I do really like Maxon, I think I prefer these set of suitors in the Heir. 
 f) Because this is a generation later, things, especially some of the phrases were modern and less backward. 
 g) At first, I found it hard to reconcile the image of America and Maxon happy and newly-wed with the still happy and loving, but grown-up, King and Queen, parents they were. But it was ok and I got used to it. I liked that the author included all the favourite, main characters from the previous books, like Miss Marlee and her husband, General Leger and Miss Lucy and Aunt May. I could see some of their personalities from before still shining through so it wasn't like the characters were unrealistic. I also liked that Eadlyn had siblings that I actually liked. The bond between Eadyln and her twin brother Ahren was really sweet and evident and that was nice to read about too. 
 h) I was warned the ending might be bad but I thought it was ok. 
 i) I liked some of the boys that Eadlyn met. She basically shows some interest in Kile, Henri, Erik, Hale and Fox. She shows more interest in Kile and Henri though and a bit too in Erik. Which makes me think she'll choose either Kile or Henri, and I like both of them and am happy either way. However, these boys were the ones who we got to know, the ones who got characterised and appeared mulitple times through the book; I think that's the only reason we at all connect to them. 
 j) Thank god, most of the Selected get along. In The Selection etc. all the girls were so annoying and ill-tempered. The boys have an easy camaraderie that was missing from the other books. 
 Though, of course, there were some things that bugged me, namely:- 
 I didn't like Eadlyn as much as America. So I suppose that's a bad thing here but a good thing in the other books. Also, I just didn't really feel The Selection like I did last time. All the boys are neutral for me. Some of them were really weird and creepy but got kicked out in the end. I don't really like Ahren anymore for what he did at the end. I thought he was the rational type??? And the insta-love between Eadlyn and Kile. I mean, wtf?! So she never got to know him, your fault Eadlyn for being such a brat. And she views him with disgust and contempt initially. But then he's in the Selection, and like two minutes later after they have one talk, she likes him, and she starts kissing him and he lets her?! Really Eadlyn, where was Kile hiding? Oh, let me see, in the palace like in a room across from you. Anyway, I just thought it was really abrupt and so fake; with Henri, on the other hand, it was slower and more build-up in the relationship. That doesn't mean I don't like Kile; I just think it could've been done better. (view spoiler)[And lol it'll be weird if America dies, though if the author knows what's good for her she probably won't kill her off ;) (hide spoiler)] 
 I think it'll be interesting how the author wraps up Eadlyn's story in the next/last book, considering that America's took three. Though Eadlyn did fasten up things quite a lot in the book. I also wonder what it's going to be called. 
 Overall, I felt that there were some problems, but not enough to really ruin the book. Does not need to be read; The Selection series is fine as it is. I just felt this book was more interesting to read than the three previous ones (if you're wondering why I still gave this 4 stars; there were good things balancing the bad things out).