[ Eq(Derivative(x1, t), (-1)*k4*k1*x2*x1/k4),
  Eq(Derivative(x2, t), (1*k4*k1*x2*x1 + (-1)*k4*k3*x2 + (-1)*k4*k2*x2)/k4),
  Eq(Derivative(x3, t), 1*k4*k3*x2/k4),
  Eq(Derivative(x4, t), 1*k4*k2*x2/k4) ]