{ diff(x1, t) = (-1)*k4*k1*x2*x1/k4,
  diff(x2, t) = (1*k4*k1*x2*x1 + (-1)*k4*k3*x2 + (-1)*k4*k2*x2)/k4,
  diff(x3, t) = 1*k4*k3*x2/k4,
  diff(x4, t) = 1*k4*k2*x2/k4 }