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