[ Eq(Derivative(x1, t), 0),
  Eq(Derivative(x2, t), (1*(k8*k2 - k1*x2) + (-1)*((-k5*x3) + k6*x2/(1 + x4)) + (-1)*((-k3*x4) + k4*x2/(1 + x4)))/k7),
  Eq(Derivative(x3, t), 1*((-k5*x3) + k6*x2/(1 + x4))/k7),
  Eq(Derivative(x4, t), 1*((-k3*x4) + k4*x2/(1 + x4))/k7) ]