ménage à quatre
/ˌmeɪnɑːʒ ɑː ˈkatrə/ /ˌmɛnɑːʒ ɑː ˈkatrə/ /məˌnɑːʒ ɑː ˈkatrə/noun
A relationship or domestic arrangement in the style of a ménage à trois, but involving four people.
Origin
1950s; earliest use found in The Oakland Tribune. From French ménage à quatre.