My teacher said that the textbook (which says よんぷん）is wrong and told us to cross it out and replace it with よんふん, but she also occasionally make other mistakes, so I'm unsure if she is correct or if the textbook is correct.

To say "four minutes" in Japanese (when telling time, like 2:04), do you pronounce it as よんふん or よんぷん?

From what I can see on the Internet this is something that is an issue even among native Japanese speakers. It appears that よんぷん is considered technically correct, but some people (possibly younger people especially) say よんふん.

In Kansai almost everyone seems to say よんふん, but I've heard that in Kanto it's pronounced よんぷん.

I don't buy that there's a 'correct way' to say 4分.