The 4°/75% is ballpark figure which keeps the aircraft flying, when you have either no airspeed indication or no idea which airspeed indication is correct.
It is not meant as a "I can still fly RVSM to destination" figure. Its sole purpose is to keep enough airflow over the wings so the aircraft continues to fly (but not so fast as to have it fall apart), while you bring out the PI figures, as suggested.