[ Eq(Derivative(x1, t), 0),
  Eq(Derivative(x2, t), (1*(k9*k2 - x2*(k1 + k3*x4)) + (-1)*((-k6*x3) + k7*x2) + (-1)*(k5*x2 - k4*x4))/k8),
  Eq(Derivative(x3, t), 1*((-k6*x3) + k7*x2)/k8),
  Eq(Derivative(x4, t), 1*(k5*x2 - k4*x4)/k8) ]