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