love quadrangle; four-way relationship
Refers to a romantic or interpersonal configuration involving four people, often with overlapping affections, analogous to a love triangle but with four participants.
That drama has such a complicated love quadrangle that I lose track of who likes whom.
三角関係 is a love triangle involving three people, while 四角関係 extends the pattern to four people.
Compound of 四角 (four corners, square) and 関係 (relationship), modeled after 三角関係 (love triangle).