Surname 四井 (Yotsui 
, 四井 
, Сии 
) , in the world, is a very rare last name. The family name 四井 is characteristic of Brazil, where it is a very rare surname, Russia, where it is an extremely rare surname, and the United Kingdom. Not in comparison with a country's population, the name is most common in Brazil, China, and the United States. More prevalently, 四井 is the forename as well as the last name.