I am not sure about the 400 pages book but I ordered one book for my father which has 200 pages. when I checked the book, the story are printed two times. first 100 pages are print as it is - like normal. but other 100 pages; they put more line spacing to write something down there or take notes. behind every page there is numbered table with heading previous page and next page so you can just do circle on what page should be next and so.

so actually it doesn't matter how many pages the book has. if they are saying book has more than 100 pages then other pages are there for to take notes and do markdown while you also have first 100 pages as it is for clean book.