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