  From near-infrared spectroscopic measurements of the MgII emission line
doublet, we estimate the black hole (BH) mass of the quasar, SMSS
J215728.21-360215.1, as being (3.4 +/- 0.6) x 10^10 M_sun and refine the
redshift of the quasar to be z=4.692. SMSS J2157 is the most luminous known
quasar, with a 3000A luminosity of (4.7 +/- 0.5) x 10^47 erg/s and an estimated
bolometric luminosity of 1.6 x 10^48 erg/s, yet its Eddington ratio is only
~0.4. Thus, the high luminosity of this quasar is a consequence of its
extremely large BH -- one of the most massive BHs at z > 4.
