From MangaHelpers:

There's an idea that your blood type affects your personality: Type-A's are serious perfectionists, Type-B's are individualistic, Type-O's are outgoing, and Type-AB's are strange. This 4-koma features the four sisters of the Amano family, who are each very different from one another in looks, personality, and even blood type. Despite growing in the same environment, they each have a personality similar to their blood type. The oldest, Type-A Asahi, carefully dresses like a stylish girl to hide her perfectionism. The second sister, Type-B Mahiru, follows her own path. The next youngest, Type-O Yuu, is very social and gregarious. The youngest sister, Type-AB Koyoru, is distant and hard to read. The four sisters go through their daily life together, each very different from the other! [tethysdust]