  The first search is presented for vector-like leptons (VLLs) in the context
of the "4321 model", an ultraviolet-complete model with the potential to
explain existing B physics measurements that are in tension with standard model
predictions. The analyzed data, corresponding to an integrated luminosity of
96.5 fb$^{-1}$, were recorded in 2017 and 2018 with the CMS detector at the LHC
in proton-proton collisions at $\sqrt{s}$ = 13 TeV. Final states with ${\geq}$
3 b-tagged jets and two third-generation leptons ($\tau\tau$, $\tau\nu_\tau$,
or $\nu_\tau\nu_\tau$) are considered. Upper limits are derived on the VLL
production cross section in the VLL mass range 500-1050 GeV. The maximum
likelihood fit prefers the presence of signal at the level of 2.8 standard
deviations, for a representative VLL mass point of 600 GeV. As a consequence,
the observed upper limits are approximately double the expected limits.
