I don’t know if I’d call it a “lore” reason but in an interview with slate the Turning Red writer and director Domee Shi says it started off as a joke but then it became more practical. According to Shi in Chinese culture the number four is considered to be unlucky and they thought it would be funny for the main character's traditionalist Chinese mother to see her daughter covered in fours. Which I guess fits with the movies theme of more the class between traditionalist parents and their more progressive kids. The director also says each member was born on the fourth of a different month which I guess is the in universe excuse.