  We present observations and analysis of an unusual [C II] emission line in
the very luminous QSO SDSS J155426.16+193703.0 at z~4.6. The line is extremely
broad (FWHM 735 km/s) and seems to have a flat-topped or double-peaked line
profile. A velocity map of the line shows a gradient across the source that
indicates large-scale rotation of star-forming gas. Together, the velocity map
and line profile suggest the presence of a massive rotating disc with a
dynamical mass M_dyn > 5x10^10 M_sun. Using the assumption of a rotating disc
origin, we employ an empirical relation between galaxy disc circular velocity
and bulge velocity dispersion (sigma) to estimate that sigma > 310 km/s,
subject to a correction for the unknown disc inclination. This result implies
that this source is consistent with the local M--sigma relation, or offset at
most by an order of magnitude in black hole mass. In contrast, the assumption
of a bulge origin for the [C II] emission line would lead to a conclusion that
the black hole is nearly two orders of magnitude more massive than predicted by
the M--sigma relation, similar to previous findings for other high-redshift
QSOs. As disc rotation may be a common origin for [C II] emission at high
redshifts, these results stress that careful consideration of dynamical origins
is required when using observations of this line to derive properties of
high-redshift galaxies.
