[ Eq(Derivative(x1, t), (1*k9*x2/k9 + (-1)*k9*k1*x1/k9)/k9),
  Eq(Derivative(x2, t), (1*k9*k5*Piecewise((sin(t)**2, Lt(t, k7*pi)), (0, True))*Piecewise((1, Lt(t, k8)), (0, True))/k9 + 1*k9*k2*x3**2/(k6**2 + x3**2)/k9 + (-1)*k9*x2/k9)/k9),
  Eq(Derivative(x3, t), (1*k9*k1*x1/k9 + (-1)*k9*k2*x3/k9 + (-1)*k9*k3*x4*x3/k9)/k9),
  Eq(Derivative(x4, t), (1*k9*x2/k9 + (-1)*k9*k4*x4/k9)/k9) ]