  The fourth catalog of active galactic nuclei (AGNs) detected by the Fermi
Gamma-ray Space Telescope Large Area Telescope (4LAC) between 2008 August 4 and
2016 August 2 contains 2863 objects located at high Galactic latitudes
(|b|>10{\deg}). It includes 85% more sources than the previous 3LAC catalog
based on 4 years of data. AGNs represent at least 79% of the high-latitude
sources in the fourth Fermi-Large Area Telescope Source Catalog (4FGL), which
covers the energy range from 50 MeV to 1 TeV. In addition, 344 gamma-ray AGNs
are found at low Galactic latitudes. Most of the 4LAC AGNs are blazars (98%),
while the remainder are other types of AGNs. The blazar population consists of
24% Flat Spectrum Radio Quasars (FSRQs), 38% BL Lac-type objects (BL Lacs), and
38% blazar candidates of unknown types (BCUs). On average, FSRQs display softer
spectra and stronger variability in the gamma-ray band than BL Lacs do,
confirming previous findings. All AGNs detected by ground-based atmospheric
Cherenkov telescopes are also found in the 4LAC.
