Four days between chafifa and tevila

The gemara in nida on 67b says has a machlokes if we allow the woman to do the chafifa two days before the tevila, three days and four days. According to one opinion we allow her to do it because there are times that we require her to do the chafifa that many days in advance. Two days when the tevila is Motzei Shabbos, three days when Motzei Yom Tov is Sunday and four days when Rosh Hashana is Sunday/Monday.

The Maharsha asks why the gemara limits it to the case of Rosh Hashana and why it didn't just continue talking about Yom Tov. Afterall, these were Amoraim living in Bavel having this discussion. He says that the Shach answers the questions. I looked in the Aruch Laner and he also asks another question: Rosh Hashana can't fall out on Sunday - "Lo Ad"u Rosh" so why does the gemara use this example? He quotes the Shach to answer both questions. The Amoraim wanted to show that included in the original takana of Ezra was the allowance to do the chafifa so far in advance. Just because nowadays it's possible for two days of yom tov to follow Shabbos, if it wasn't possible in the times of Ezra then that wouldn't prove that the chafifa could be done in those situations so far in advance. That's also why Rosh Hashana could be on Sunday. Even in the days of Ezra they were careful that Rosh Hashana wouldn't fall out on Wednesday or Friday so Yom Kippur shouldn't be back to back with Shabbos but they were not careful that it shouldn't fall out on Sunday. It wasn't a huge deal if Hoshana Rabba fell out on Shabbos. Even though nowadays it can't happen because we'd prefer if Hoshana Rabba didn't happen on Shabbos it wasn't impossible back then.