  The thermal conductivity of monolayer graphene is an outstanding challenge
with no consensus reached on its exact value and length convergence so far. We
consider four-phonon scattering, phonon renormalization, and an exact solution
to phonon Boltzmann transport equation (BTE) from first principles. Using this
computational formalism with unprecedented sampling grid, we show that when
four-phonon scattering is included the thermal conductivity is convergent with
system size at a room temperature value of 1300 W/(m$\cdot$K), which is lower
than that of diamond. On the contrary, considering three-phonon scattering only
yields divergence with size due to the momentum-conserving normal processes of
flexural phonons.
