P{X=1}=(λ^1)/(1!) * e^-λP{X=2}=(λ^2)/(2!) * e^-λ解得λ=2（λ=0不符合λ>0的性质，舍去）P{X=4}=2^4/4! * e^-2 = 2/3 * e^-2P{X>1} = 0.593994（查表）
