While I don't think there's anything wrong with the idea of studying for several hours without a break, if that method isn't working out for you, it isn't working out. Don't concentrate on the method - concentrate on the result. You want to learn something - why does it matter whether you learn it through 4 hours of uninterrupted reading, 5 hours with breaks, or something else? If the uninterrupted reading idea isn't working out for you, try something else.
Even if you do manage to get yourself to sit down with a book for 4 hours, there's a good chance you won't be spending that time very efficiently - it's easy to just start skimming paragraphs or forgetting what you just read if you're not concentrating, and it's difficult to just force yourself to concentrate. Trying a different approach is probably a better idea. Soner Gönül's answer gives a lot of useful information on possible approaches.
My personal textbook-reading-motivation trick from college, rather silly but it did work: I'd let myself take long breaks, but I'd only let myself have chocolate (or whatever else snack I felt like) while I was reading the textbook, not doing other things. It ended up being enough of a temptation to get my studying done. ;)