(UPD. Disregard the first paragraph - haven't read a single old-fashioned paper book in the last decade, I missed the fact that they are printed on both sides of the pages. Looking through 4 overlaying texts at once would probably turn them into an unintelligible mess. The idea with a book scanner may still work, though...)
If those books have strict and unchanging requirements about fonts, margins size etc., and if the content doesn't change much between editions, would it be possible to compare by overlaying corresponding pages on top of a glass panel with a lamp underneath? Sort of similar to how animated movies were drawn before the computers, but in reverse? You would notice immediately if the pages are identical. If there's a change in the middle of one of the pages, you would also see it. Of course, then the rest of that page (or even chapter) wouldn't match, even for identical texts, but you might be able to shift one of the pages to align the next paragraphs and continue.
If the idea works, you may try a more technologically advanced variation of it using a book scanner (not necessarily that model; then you compare two semi-transparent page images on your computer screen instead, may be able to automate that some more, e. g. cut out paragraphs, highlight significant differences, etc.). If it doesn't, then maybe add some sort of OCR into the process?