Maximize[50 x1+30 x2+70 x3+80 x4,{3 x1+2 x2+5 x3+4 x4<=600,2 x1+x2+3 x3+4 x4<=400,x1+x2+2 x3+3 x4<=300,x1<=90,x2<=120,x3<=70,x4<=50,x1>=0,x2>=0,x3>=0,x4>=0},{x1,x2,x3,x4},Integers]