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