Up until today, I used to think identical twins were always the same sex (boy-boy or girl-girl) and fraternal twins were mixed (boy-girl), but wikipedia has disabused me of that incorrect notion.
So I guess I'll just call them opposite-sex twins from now on.
Fraternal twins can be either two girls, two boys, or one of each. In very rare cases, identical twins can be different sexes.
@Yosha_x is it this one? https://mangadex.org/title/16972/kekkon-shite-mo-koi-shiteru
Not 4-koma, but the family fits the description (and it's a cute series haha <3)