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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., Comment: arXiv admin note: text overlap with arXiv:1811.01570, Accepted by ApJS

"Changing-look" active galactic nuclei (CL-AGNs) are a newly-discovered class of AGNs that show the appearance (or disappearance) of broad emission lines within a short time scale (months to years) and are often associated with the dramatic change of their continuum emissions. They provide us an unprecedented chance to directly investigate the host galaxy properties with minimal contamination from the luminous central engine during the "turn-off" state, which is difficult for normal luminous AGNs. In this work, for the first time, we systematically characterize the stellar populations and star formation histories (SFHs) of host galaxies for 26 turn-off CL-AGNs using the stellar population synthesis code STARLIGHT. We find that the stellar populations of CL-AGNs are similar to that of normal AGNs, excepts that the intermediate stellar populations contribute more fraction. We estimate their stellar velocity dispersions ($\rm \sigma_{\star}$) and black hole masses ($\rm M_{BH,vir}$) and find that CL-AGNs also follow the overall $\rm M_{BH}-\sigma_{\star}$ relationship. We also confirm the previous claim that CL-AGNs tend to be biased towards lower Eddington ratio, and their extreme variabilities are more likely due to the intrinsic changes of accretion rates. In addition, CL-AGNs with recent star formations (SF) tend to have higher Eddington ratio. Compared with previous studies, our analysis suggests that there may be a correlation between the CL-AGN host galaxy properties and their CL phenomena., Comment: accepted by ApJ