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