Most series tend to lose steam as the number of sequels increases. The first two series with 4 seasons that come to mind are Natsume Yuujinchou, which just kind of started repeating itself after about 2 seasons, and Minami-ke, which was passed from studio to studio and changed directors, with disastrous results. Even Tamayura, a series that I love extremely dearly, started retreading old ground in the second TV season. This trend has its exceptions, though. Aria and Monogatari are two series that, in my opinion, get better and better with every sequel, at least so far.
The same is not exactly true for Hidamari Sketch, but that’s because it doesn’t have to be “either or”, nor does it have to be a case where every season is approximately equal in quality. In the case of Hidamari Sketch, it’s because the series is highly achronological. It’s a slice of life series to the highest degree, and all the individual slices have been shuffled. The first half of an episode can take place a year after the second half, for instance. This does make for a slightly(read: very) confusing experience at first, but I got used to it.
However, that’s not to say that the show keeps going in circles without making any progress. Although slowly, the story of Hidamari Sketch does advance. And it advances well. Although Yuno is usually the main focus when it comes to character development, Sae and Hiro get a ton of their own, especially later on(culminating in the Graduation Arc). Sae’s struggle with her writing and Hiro’s obsession with wanting to feel needed make them very relatable.
And then there’s Miyako, who is the source of most Hidamari-related memes(except for “Yuno what”). You’d think she’d be the eternal butt-monkey(no, that’s usually Yuno) or the character who ruins meaningful moments by being silly(…actually, it’s Yuno again, though not very often), but no, Miyako is remarkably tolerable in her eccentricity. Sure, she does silly things that make scenes silly, but there’s also definitely another side to her. She can be deeply introspective, for instance in the episode with the cat. In the 4th season in particular, Miyako gets a number of “serious scenes”, all of which are very rewarding. And that’s actually the show in a nutshell: Surprisingly rewarding, at least for a slice of life show, and surprisingly good at being serious.