We report on the room-temperature wafer-bonded InGaP/GaAs//InGaAsP/InGaAs four-junction solar cell grown by all solid-source molecular beam epitaxy ( MBE). The material growth and structure optimization of multi-junction solar cell were investigated. The effect of growth temperature on phosphide-related material of the solar cell was discussed. The lattice-matched InGaP/GaAs tandem cell on GaAs substrate and InGaAsP-or InGaAsP/InGaAs-tandem cell on InP substrate were grown separately by MBE. Room-temperature wafer bonding technique was used to integrate the subcells into multi-junction solar cells. The current limiting of multi-junction cells was analysed and the device structure of four-junction solar cell was optimized. The best wafer-bonded four-junction solar cell reached a conversion efficiency of 42% under concentration.