[ Eq(Derivative(x1, t), (1*x5*x4 + (-1)*k3*x1)/k8),
  Eq(Derivative(x2, t), (1*x6*x4 + (-1)*k4*x2)/k9),
  Eq(Derivative(x3, t), (1*k3*x1 + 1*k4*x2 + 1*k10*k5 + (-1)*k10*k6*x3)/k10),
  Eq(Derivative(x4, t), ((-1)*x5*x4 + (-1)*x6*x4)/k7) ]