dx1/dt=-(k21+k31+k41+k01)*x1+k12*x2+k13*x3+k14*x4+u1;
dx2/dt=k21*x1-k12*x2;
dx3/dt=k31*x1-k13*x3;
dx4/dt=k41*x1-k14*x4;
x1(0)=1;
x2(0)=1;
x3(0)=1;
x4(0)=1;
y1=x1

