The　existence　and　bifurcations　of　the　subharmonic　orbits　for　four-dimensional　non-autonomous　nonlinear　systems　are　investigated　in　this　paper.　The　improved　subharmonic　Melnikov　method　is　presented　by　using　the　periodic　transformations　and　Poincar,　map.　The　theoretical　results　and　the　formulas　are　obtained,　which　can　be　used　to　analyze　the　subharmonic　dynamic　responses　of　four-dimensional　non-autonomous　nonlinear　systems.　The　improved　subharmonic　Melnikov　method　is　used　to　investigate　the　subharmonic　orbits　of　a　simply　supported　rectangular　thin　plate　under　combined　parametric　and　external　excitations　for　verifying　the　validity　and　applicability　of　the　method.　The　theoretical　results　indicate　that　the　subharmonic　orbits　can　occur　in　the　rectangular　thin　plate　with　1:1　and　1:2　internal　resonances.　The　results　of　numerical　simulation　also　indicate　the　existence　of　the　subharmonic　orbits　for　the　rectangular　thin　plate,　which　can　verify　the　analytical　predictions.