[ Eq(Derivative(x1, t), k1 - k6*x1*x3 - k2*x1 + (k5 + k8)*x4),
  Eq(Derivative(x2, t), k3*x1**2 - k7*x2),
  Eq(Derivative(x3, t), k4*x2 - k6*x1*x3 + (k5 + k9)*x4 - k8*x3),
  Eq(Derivative(x4, t), k6*x1*x3 - (k5 + k9)*x4 - k8*x4) ]