  A search for the pair production of the lightest supersymmetric partner of
the top quark, the top squark ($\tilde{\mathrm{t}}_1$), is presented. The
search targets the four-body decay of the $\tilde{\mathrm{t}}_1$, which is
preferred when the mass difference between the top squark and the lightest
supersymmetric particle is smaller than the mass of the W boson. This decay
mode consists of a bottom quark, two other fermions, and the lightest
neutralino ($\tilde{\chi}^0_1$), which is assumed to be the lightest
supersymmetric particle. The data correspond to an integrated luminosity of 138
fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of 13 TeV
collected by the CMS experiment at the CERN LHC. Events are selected using the
presence of a high-momentum jet, an electron or muon with low transverse
momentum, and a significant missing transverse momentum. The signal is selected
based on a multivariate approach that is optimized for the difference between
$m(\tilde{\mathrm{t}}_1)$ and $m(\tilde{\chi}^0_1)$. The contribution from
leading background processes is estimated from data. No significant excess is
observed above the expectation from standard model processes. The results of
this search exclude top squarks at 95% confidence level for masses up to 480
and 700 GeV for $m(\tilde{\mathrm{t}}_1) - m(\tilde{\chi}^0_1$) = 10 and 80
GeV, respectively.
