"A" definitely my favorite episode as well, for all the reasons you stated. The flashbacks worked beautifully as a juxtaposition of Rick settling down into the calmer and quieter life as a farmer at the prison and him coming face-to-face with sheer brutality and accepting that in the present. It was just the perfect episode for me.
As for season 4... it was certainly no season 2. I have to say I probably enjoyed it much more than season 3, but it was really as hit and miss as this season has been. The premiere was slower, but it was a reflection of the life everyone (especially Rick) had settled into. He didn't even want to bring his gun with him when he went to check the snares, and it was an interesting contrast with the flashbacks in the finale when he would bring his gun even when Hershel told him he wouldn't need it (and remember that it was Hershel in the premiere that urged him to take his gun again). Scott Gimple has, in my opinion, undoubtedly done wonders for the show. It does feel much more cohesive and it is plain that a lot of planning was done in advance for the entire season. One example was what I said about the gun thing. Also in the premiere was Clara's dying speech and then Hershel's talk with Rick afterwards, which was all repeated by Rick as he's trying to convince the Governor to stand down and live with them. A nice little connection, which isn't as big as others, but I liked it, was Carl in the premiere telling Michonne that his sheriff hat not being a farming hat and then Rick telling him that "we're gonna have to get you a farming hat".
Episode 2 I found entertaining. There was that introduction to Lizzie's dependence on flowers that came to play a big role 12 episodes later. I really liked the ending and, I don't know why, but I really like how they took some random redshirt who nobody knew (David) and made him into a huge plot point for the entire season, to the point where people all know his name, but probably couldn't point him out alive in episode 1 (at least I can't, lol). Tyreese's reaction in the next episode was great, but, other than that, I really remember it.
So at that point season 4 was doing okay, though I know there are many who would say that the flu story arc was boring. I didn't find it boring, but I did find it pointless in the long run since it was just kicked under the carpet after episode 5 and really not mentioned again until exposition called for it in episode 14 when Carol confesses to Tyreese. Episode 4 I feel like was an episode that could be hated, but I personally loved it, and it was one of my favorites of 4A, though this may have been because they finally got out of the prison for an episode after an entire season. But I did love the conversations between Rick and Carol, their talk about their families and the discussion about the murders. We meet Sam, another loose end that was nicely tied up eventually (though Ana is still a mystery to me). 405 was a less divisive good episode, but after that, the season kind of fell down for two episodes. The Governor episodes were nice exposition and were mildly entertaining, but I did not like their overall feel for the season. Then, no question, "Too Far Gone" was amazing.
The second half is where the show started getting wonky since it was testing out a storytelling technique that hadn't been done before: episodes focusing on split up groups, some focusing on only one group. "After" was definitely a slow one after 408, and it was a hit or miss like "Indifference". "Inmates" was meh, but we did get a cool Carol return and Abe and co. intro. "Claimed" was where I believed the show picked up because I thoroughly enjoyed Rick's run-in with the Claimers in the house and was very intrigued by the whole Eugene situation with him claiming to know the cure. The scenes with that group were certainly entertaining.
And then there was "Still". Oh God fuck, "Still". That was painfully bad. That was a painfully low episode, especially when you consider this is no longer season 2 and we've been with Scott Gimple-quality episodes for a while. I'm not sure where the quality went but I feel like there was a clear drop in one judging by the fact that "Still" is one of the lowest rated, if not THE lowest rated, episode on IMDb.
"Alone" was all right. I liked the Bob flashback in the beginning but the rest of the episode was kind of boring to me, particularly the Bob/Maggie/Sasha parts, though the Daryl/Beth scenes were not that compelling. Most interesting parts were Beth's kidnapping and Daryl's recruitment by the Claimers at the end.
"The Grove" -- no question about it. Definitely up there with "Too Far Gone" and "A". The whole Lizzie situation kept me hooked to the show, and the whole back and forth between Carol and Tyreese kept me on my toes; you could always feel Carol on the verge of telling Tyreese but she never did, not until the end at least. And then the deaths.... It was all amazing.
"Us" was all right. Daryl's involvement with the Claimers was entertaining and I liked the Glenn and Abe and co. scenes, especially the one in the tunnel and the reunion afterward. And then there was "A" after that.
To me, season 4 was a huge leap up from previous seasons (excepting season 1, in my opinion). It and season 5 have felt so much richer than past seasons and I do believe it has been because of Gimple's direction with the show. He's brought us some great stuff and I can't wait to see where he brings us. I do believe that they've found a keeper.