Penta-graphene (PG), as a novel and stable pentagonal structure, has attracted considerable attention due to its large band gap, ultrahigh strength, negative Poisson's ratio, which indicates promising potential in nanoelectronics and nanomechanics. Thermal conductivity is a key physical quantity for the potential applications of PG based nano-devices. However, previous studies focused on the thermal conductivity of PG dominated by the well-known cubic anharmonic effect, and the quartic anharmonicity, although probably significant, was still neglected. In this work, we study the effect of four-phonon scattering on the thermal transport in PG through using first-principles calculations and solving the Boltzmann transport equation (BTE). We find that the intrinsic lattice thermal conductivity of PG is 182.1 W/mK at room temperature after including four-phonon scattering, which is reduced by 73.5 % as compared to the obtained results with only three-phonon scattering. Meanwhile, the relative contribution of the flexural acoustic (ZA) branch is reduced from 60.5 % to 32.5 % when four-phonon scattering is included. Thus, the large reduction in thermal conductivity can be ascribed to the large four-phonon scattering rates, which are completely comparable with those of three-phonon processes in the acoustic phonon frequency range even at 300 K. It is well believed that this finding is not a special case but should exist in other 2D materials with similar phonon features. Furthermore, the validity of the sum of three- and four-phonon scattering rates has been verified by the results based on the MD simulations. Our finding provides critical revisit and valuable insight to the exact thermal conductivity value of PG.