[ Eq(Derivative(x1, t), 0),
  Eq(Derivative(x2, t), 0),
  Eq(Derivative(x3, t), (2*k2*k6*x4 + (-1)*k3*x3**2 + (-1)*k4*x3*x4 + (-1)*k5*x3)/k1),
  Eq(Derivative(x4, t), ((-1)*k2*k6*x4 + 1*k3*x3**2)/k1) ]