our program faculty have advised us to use at least 3 different books to expose ourselves to different ways of asking questions and to help with preparation for nclex...
i've used saunders (i find them to be lower level), lippencott (more challenging), davis (more challenging) and hogan (more challenging but annoying - i find errors and the cd is not set up well - it's really only useful for the practice exam of 265 questions - i would not buy this book again)...we also do hesi case studies and meds pub as part of our program
i can only think of 4 major ones, too - saunders, kaplan, lippencott and davis but adding in mosby would be the 5th hehe
good luck to you