  Young multiple star systems provide excellent testing grounds for theories of
star formation and evolution. EPIC 203868608 was previously
studied~\citep{David2016} as a triple star system in the Upper Scorpius OB
association, but the follow-up Keck NIRC2/HIRES/NIRSPAO observations reported
here reveal its quadruple nature. We find that the system consists of a
double-lined spectroscopic binary (SB2) Aab (M5+M5) and an eclipsing binary
(EB) Bab with a total mass that is lower than that of the SB2. Furthermore, we
measure the obliquity of the EB using the Doppler tomography technique during
the primary eclipse. EPIC 203868608 Bab is likely on an inclined orbit with a
projected obliquity of $-57^{+40}_{-36}$ degrees. The inclined orbit is used to
constrain the tidal quality factor for low-mass stars and the evolution of the
quadruple system. The analytic framework to infer obliquity that has been
developed in this paper can be applied to other EB systems as well as
transiting planets.
