1. The book that I borrowed from Frank was very interesting. The word "which" in number 2 is incorrectly used. Which is a word used to choose between things or people. Number 3 indicates that borrowing a book was the interesting thing. The book was not the interesting thing. Number 4 is similar to 3 and indicates that borrowing a book from Frank was the interesting thing. Once again the book is not the interesting thing. Only the first is correct.

All are correct, except for number 2, where "which" should be "that". Mina_orange said that 4 and 3 mean that the act of borrowing a book from frank is interesting. This isn't strictly correct. I don't see how this could be the case for number 3 at all. But as for number 4 the tone of the speaker would indicate the meaning. If there is a slight pause after "Frank" and then the word "that" was stressed that would mean that they thought the act of borrowing the book was interesting. However, if there is no pause after frank and a rising intonation on the word "interesting" it would mean the same as the other sentences - that the book was interesting. It's hard to say which is the most natural as each sentence could be deemed the most natural depending on the context of the conversation. I hope this helps.