Looking on the first two seasons especially, it does give impression that Osomatsu-san leans to Comedy the most, while sometimes can also have its deep emotional moments. Season 3 introduces storyline (inclusion of Omusubi/Riceballs) where this can be a hit or miss as a following season. The reason being the need to keep up / try to not miss details as much as possible. After giving myself a rewatch, Season 4 feels more tied with this thing where you have to watch the episodes in order since they are connected each other (not all, but most episodes are), so that's something to keep in mind.It's also noticeable that the tone for this season leans to more peaceful situation and wholesome kind of ending. There is a sense of growth as the series has been going for around 10 years, but also sense where the media feels 'detached' from other seasons since the sextuplets are barely wearing their usual hoodies / daily outfits with different colors, and with how the story seems to not really highlight the character's 'known' personalities. And so if you've known the characters enough and doesn't mind such details, Season 4 will work for you, otherwise it most likely won't.

Now coming to the more detailed points (mostly what I feel when watching the anime): The storyline feature for this season, sometimes has an awkward pacing where a scene can take too long and missing some kind of variety, the same goes to BGM usage per episode. It's not the kind of season where you can rewatch your favorite ones for several times, they are not as memorable as the previous ones and meant to watch for the story. Asiding those things however, I seem to be the side who enjoys this more wholesome bits and the unusual side of Osomatsu-san characters.
Taking an example [slight spoiler warning]: Jyushimatsu, his more serious side may seem unusual due to the most known impression of being extraordinary, but this has been built even from several episodes in Season 1. He mostly will become the first out of everyone to notice if something is odd, and he will be the straight man whenever it feels appropriate (a good example being first half in Season 3 Episode 5). With this side he's still Jyushimatsu, he's still doing more movements as ever and his signature open mouth is still visible enough. Being put with the tone for this season, it does feel less than usual, but it's not entirely different and made those different sides to shine.

Each season of Osomatsu-san has its own kind of flavor. And this may feel wrong to write on an anime being put as Comedy genre, but Osomatsu-san doesn't have to be a 'comedy' and it's not a weird thing for the sextuplets to get along. Definitely agree with how the execution could have been better though. As for the OP and ED, songs are nice and seems fitting for the anime. The use of MMD instead of usual wackiness is definitely something (some said that it brings back to fanmade MMD content era). I especially really like the ending song, it's nice to see some kind of MV with Totoko and the sextuplets.

As I currently explore more Osomatsu-san contents, I look forward for Episode 13 and the upcoming seasons if it ever exists. For this rating I'm putting it on mixed since it entirely depends on the kind of enjoyment you have (and it's not something I can highly recommend, probably need to be a long-term or related content kind of fan).