[ Eq(Derivative(x1, t), ((-1)*k1*k2*x1 + (-1)*k1*(k3*x2*x1 - k4*x4))/k1),
  Eq(Derivative(x2, t), (1*k1*k2*x1 + (-1)*k1*(k3*x2*x1 - k4*x4) + 2*k1*k5*x4)/k1),
  Eq(Derivative(x3, t), (1*k1*k2*x1 + 1*k1*k5*x4)/k1),
  Eq(Derivative(x4, t), (1*k1*(k3*x2*x1 - k4*x4) + (-1)*k1*k5*x4)/k1) ]