  We report the fourth installment in the series of the Large Sky Area
Multi-Object Fiber Spectroscopic Telescope (LAMOST) Quasar Survey, which
includes quasars observed between September, 2017 and June, 2021. There are in
total 13,066 quasars reliably identified, of which 6,685 are newly discovered
that are not reported in the SDSS DR14 quasar catalog or Million Quasars
catalog. Because LAMOST does not provide accurate absolute flux calibration, we
re-calibrate the spectra with the SDSS/Pan-STARRS1 multi-band photometric data.
The emission line properties of H$\alpha$, H$\beta$, Mg\,{\sc ii} and C\,{\sc
iv}, and the continuum luminosities are measured by fitting the re-calibrated
spectra. We also estimate the single-epoch virial black hole masses ($\rm
M_{BH}$) using the derived emission line and continuum parameters. This is the
first time that the emission line and continuum fluxes were estimated based on
LAMOST re-calibrated quasar spectra. The catalog and spectra for these quasars
are available online. After the nine-year LAMOST quasar survey, there are in
total 56,175 identified quasars, of which 24,127 are newly discovered. The
LAMOST quasar survey not only discovers a great number of new quasars, but also
provides a database for investigating the spectral variability of the quasars
observed by both LAMOST and SDSS, and finding rare quasars including
changing-look quasars and broad absorption line quasars.
