[ Eq(Derivative(x1, t), 1*(k3*x3 - k4*k5**k6*x1)/k2),
  Eq(Derivative(x2, t), 1*(k7*k8**k9*x3 - k10*x2)/k2),
  Eq(Derivative(x3, t), ((-1)*(k3*x3 - k4*k5**k6*x1) + (-1)*(k7*k8**k9*x3 - k10*x2) + (-1)*(k11*x3 - k12*x4))/k2),
  Eq(Derivative(x4, t), 1*(k11*x3 - k12*x4)/k2) ]