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