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