4-koma is a format that generally sticks to one genre: comedy. That's not to say that no one else has tried using it for more serious fare, or that they haven't made it work for them and not against them.
SHOULDER-A-COFFIN KURO (Hitsugi Katsugi no Kuro ~ Kaichu Tabi no Wa), by Satoko Kiyuoki. First published in 2004 and first published in North America in 2008.
Kuro is a lone traveler, a witchy-looking girl accompanied by a talking bat, a coffin strapped to her back, and a lot of secrets. As she travels the land, she helps those she encounters along the way with their problems. She even (reluctantly) takes in a couple of child-like catgirls on their quest to find their 'father.' No one knows the reason Kuro is travelling, but what they do know is that her quest is growing less and less lonely by the day.
Shoulder-A-Coffin Kuro is a true oddity even for a 4-koma manga. It's not a comedy, but instead a wistful drama. It doesn't read like a 4-koma for the most part. Honestly, it's debatable if this needed to be a 4-koma in the first place. In spite of that, the story works because Kiyuoki takes the story and the characters within it perfectly seriously.
That's not to say that there isn't any humor. There are plenty of little comic moments tucked between the larger story arcs, and these play out in a fashion that's more familiar to 4-koma readers. For the most part, though, this isn't a comedy but instead the tale of a single woman improving the lives of those around her as she travels the world. It's not trying to build upon gags as it is building up the mystery behind Kuro and her journey as well as the emotional arcs of those around her.
Kuro herself is a mystery wrapped up in reluctant, somewhat wry blackness. While she is helpful and empathetic towards others, she's not purposefully seeking to help others. It's just that so often helping others helps her either move on to the next town or get her something that she needs. She's also a rather lonely girl, although this is mostly self-imposed for reasons not yet explained. She doesn't feel a need for friends or travelling companions, which makes it all the more ironic when she takes in what are basically a couple of toddlers with cat ears. Of course, being precocious little dears the two end up worming their way into her heart and she starts treating them less like little cat-eared burdens and more like family. Thus she manages to achieve an emotional arc while remaining a mystery thorough.
She's far from the only character to get one, though. The catgirls, Sanju and Nikiju, have one as well. They were the creations of a mad scientist who like Kuro just fine, but they always keep expecting their 'father' to come back someday. By the end, Kuro's distance combined with their child-like impatience leads them to run away, and when Kuro finds them again the two finally accept that Kuro is their parental figure for better or worse. Sanju and Nikiju could have been just tokens, but instead they get an emotional arc all of their own. Even some of the other incidental characters get something of one when we get to see them have a sort of reunion close to the end of the volume. Most of them were plagued by loneliness and awkwardness before, but now not only can they connect with those in their own communities, but can reach out to similar folk whose lives were also touched by Kuro. It's not only a nice callback for the reader, but it gives the volume a sense of closure despite being an ongoing story. I may never understand why Kiyuoki chose this particular format for her story, but she puts more than enough effort into her story and characters that the format ultimately doesn't matter.
Even for a manga, Kuro's artstyle is very stylized. All of the characters are practically chibi with their giant head and thin, almost unfinished limbs. Their faces are mostly dominated by their enormous round eyes and their hair tends to be a collection of limp points. Yet this heavily stylization works. It gives the story another layer of whimsy and helps to soften some of the harder edges of the story. Every character is unique - even Sanju and Nijiku have enough differences that you can tell one girl from the others despite being twins. She even manages to get a lot of expression through despite the fact that faces here aren't really built for a lot of subtle acting.
Kiyuoki also doesn't slack off on the setting. Backgrounds are reasonably elaborate despite the small size of the panels, but never to the point of distracting from the real action. Best of all, every house and alleyway is different from one other; every village Kuro and co. come upon has enough little differences to distinguish itself from the others. Again, though, this just begs the question of why she used the 4-koma format in the first place. Even the coloring is interesting, as even the brightest, sunniest scenes have a sort of sepia tone to them. Normally this would indicate something like a flashback, as the color tends to evoke old photography for most. Here it's used more like an emotional dampener. It keeps these scenes from looking or feeling too bright, as if Kuro's presence dims them a little just with her mere presence. If that's a purposeful choice, then it's a rather clever one.
Shoulder-A-Coffin Kuro has an artstyle and a format that would suggest whimsy but it's got more than enough character and emotion to it to make it a surprisingly effective drama. It's an odd but underrated little gem of a series.
This series is published by Yen Press. This series is ongoing in Japan with 5 volumes available. All 5 are currently available and are currently in-print.