  FourPhonon is a computational package that can calculate four-phonon
scattering rates in crystals. It is built within ShengBTE framework, which is a
well-recognized lattice thermal conductivity solver based on Boltzmann
transport equation. An adaptive energy broadening scheme is implemented for the
calculation of four-phonon scattering rates. In analogy with $thirdorder.py$ in
ShengBTE, we also provide a separate python script, $Fourthorder.py$, to
calculate fourth-order interatomic force-constants. The extension module
preserves all the nice features of the well-recognized lattice thermal
conductivity solver ShengBTE, including good parallelism and straightforward
workflow. In this paper, we discuss the general theory, program design, and
example calculations on Si, BAs and $\mathrm{LiCoO_2}$.
