  The observation of the production of four top quarks in proton-proton
collisions is reported, based on a data sample collected by the CMS experiment
at a center-of-mass energy of 13 TeV in 2016-2018 at the CERN LHC and
corresponding to an integrated luminosity of 138 fb$^{-1}$. Events with two
same-sign, three, or four charged leptons (electrons and muons) and additional
jets are analyzed. Compared to previous results in these channels, updated
identification techniques for charged leptons and jets originating from the
hadronization of b quarks, as well as a revised multivariate analysis strategy
to distinguish the signal process from the main backgrounds, lead to an
improved expected signal significance of 4.9 standard deviations above the
background-only hypothesis. Four top quark production is observed with a
significance of 5.6 standard deviations, and its cross section is measured to
be 17.7 $^{+3.7}_{-3.5}$ (stat) $^{+2.3}_{-1.9}$ (syst) fb, in agreement with
the available standard model predictions.
