[ Eq(Derivative(x1, t), 0),
  Eq(Derivative(x2, t), (1*k2*k3*k8 + (-1)*k2*x4*k5*x2/(k4 + x2) + (-1)*k2*k7*x2)/k2),
  Eq(Derivative(x3, t), (1*k2*x4*k5*x2/(k4 + x2) + (-1)*k2*k6*x3)/k2),
  Eq(Derivative(x4, t), 0/k2) ]