In the long run, HD 91962 is dynamically stable. This unique quadruple star system probably formed from the collapse of a rotating core of gas and dust. The rotation prevented the core from collapsing directly, causing the gas and dust to form a massive and unstable disk around the central star. The disk fragmented into a number of companion stars that migrated in towards the central star. The first few companion stars probably merged with the central star, while the companion stars that formed further out stopped their inward migration when the disk of gas and dust eventually dissipated. Hence, the 3 companion stars observed today are the ones that have survived.
Andrei Tokovinin, David W. Latham & Brian D. Mason (2015), “The unusual quadruple system HD 91962 with a "planetary" architecture”, arXiv:1504.06535 [astro-ph.SR]