The new writers will give season four some new blood. However, I don't know their work and haven't researched them, so I have no real opinion of them.

As for the writers coming back:

1. Meghan McCarthy: She has a very well-done, extensive résumé in the form of several popular episodes like Dragonshy, Party of One (my favorite season one episode for being so well-written and well-characterized), Lesson Zero, Sweet & Elite, A Canterlot Wedding, and so on. She's done well as a writer, but has a spotty record keeping the team from collectively rushing out many of season three's scripts. (One Bad Apple, Spike at Your Service, Keep Calm and Flutter On, Games Ponies Play, and Magical Mystery Cure had either moments where the script was poorly paced or had the pace crammed from beginning to end. The Crystal Empire, Part 2's pace crawled, and Equestria Girls was a rushed mess.) With a bigger budget, only time will tell if McCathy can hone her editing skills and perform better.
   
2. Dave Polsky: The episodes he's written have been decent to questionable at best despite being able to write the characters well and blend in some of his unique sense of humor into the characters organically. Feeling Pinkie Keen, despite being a fine episode for the most part, had a very questionably written friendship report that left way too many people scratching their heads. Over a Barrel took a very sensitive concept (a parody of the Natives and settlers) and blew it.
    
   Too Many Pinkie Pies's and Keep Calm and Flutter On's characterizations excelled. (TMPP explored Pinkie's determination to make things right and show how she's not a bumbling, random airhead. If given another twenty minutes, KCaFO would've been more memorable for the right reasons and not for Discord's redemption being crammed and poorly paced.) Games Ponies Play is arguably the worst episode last season and among the worst in the entire series. (I'd argue it's worst than The Mysterious Mare-Do-Well, but you can toss a coin on that one.) We'll see what he can do in season four.
   
3. Amy Keating Rogers: It was speculated she was going to come back, and now it's confirmed! *yay* With Larson not among the list of writers, she's the best one so far. Some of her episodes are extremely memorable. (The Best Night Ever, The Last Roundup, Fall Weather Friends, and Applebuck Season are highly popular, and The Smile Song is one of the best songs in the entire series due to its simplicity and memorability.) MMMystery on the Friendship Express and A Dog and Pony Show were both full of blah (the former for making the characters unnecessarily out of character to move the plot, the latter for making the Diamond Dogs so stupid in order to make Rarity the better and more intelligent character), but when she writes well, it's glorious. Good to see her back and hope she can keep it consistent.
   
4. Corey Powell: Despite writing Spike extremely out of character in Just for Sidekicks, she has the talent to write well. Pace was a common problem area in seson three; neither of her episodes had that, and JfS was loaded with content. But her debut episode, Sleepless in Ponyville, was a joy to watch. Great humor. Great storytelling. Excellent characterization. (Rarity was written very well, teasing Sweetie Belle by treating her as both her little sister and equal sister. Luna being a confident dreamwalker showed how much she grew since Luna Eclipsed, all the while without crossing moral boundaries. Rainbow Dash was at her best since Hurricane Fluttershy.) She can write some excellent episodes, and I can't wait to see her talent grow.
   
5. Charlotte Fullerton: AKR was one of the writers absent for season three. Fullerton was the other. She worked a very minute load in season two, being credited for three episodes (May the Best Pet Win!, which wasn't very good; Baby Cakes, which is actually quite good; and the story idea for Putting Your Hoof Down, which Williams wrote her second-best in). However, two of her episodes is completely memorable: Look Before You Sleep for the wonderful way Applejack and Rarity settled as foils (and developing their characterizations in the process) and Suited for Success for the well-written plot, climax, characters, and Art of the Dress. Good to see her back and hope she can perform.
   
6. Merriweather Williams: The most controversial writer currently in the lineup due to her horrible start in the form of The Mysterious Mare-Do-Well, an episode so awful that it nearly ruined Rainbow Dash's reputation. Her next episodes, Hearth's Warming Eve and Putting Your Hoof Down, did a great job from a storytelling perspective, but still kept that stigma of writing characters as unlikeable bastards in order to make the story work. (Dragon Quest, one of my favorites, also did it, but with even less success with a questionable ending. The mantra, "Actions by a specific few don't always mean they're the actions of the whole," applies here, spoiling the moral and inadvertently undermining Spike's character development.)
    
   Surprisingly, season three used less cynicism. Wonderbolts Academy was the best Dash-centered episode since Sonic Rainboom, but Spike at Your Service stunk due to Spike being contradictorily incompetent (among other reasons). That said, she's a good writer who has since been able to hone her skills and perform better. I wonder what she'll do next.
   

As for the new writers, welcome aboard the brony train! You have the résumés and talent. You showed it in your other work. Now prove it in My Little Pony: Friendship Is Magic. Good luck to all of you!

Yeah, it's a shame M. A. Larson and Cindy Morrow aren't going to write episodes for the upcoming season (their recent tweets confirmed it), but there's a good team who can churn out good episodes if given the right concept, budget, guidance, understanding, and leeway to do it. Good luck, Larson and Morrow, and we'll (hopefully) see you in season five.

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Source: List of writers for Season 4