My best understanding is that these 4 fasts are positive Rabbinic commandments. Positive commandments are governed by “Deracheha darchei noam” (Proverbs 3:17) and “Haemet V’hashalom ehavu” (pleasant ways and ways of truth and peace) as brought down in Tractate Succos (daf 32a about the lulav and 32b about the hadas). Because of this, certain types of palm branches that can scratch the skin are invalid for the Lulav. While fasts are intrinsically a day long denial of pleasure, when they can damage the body (like the prickly Lulav) there is no longer the Mitzvah to fast. We certainly have no obligation to experience Auschwitz even for a few hours.
Most interesting is that “Haemet V’hashalom ehavu” are the last words of the verse that we learn about these 4 fasts (Zechariah 8:19).
Ask your local Rabbi for specific details when the exemption applies.