I have visited Dachau twice, and four hours is enough to visit. For my visit in the future, I plan to allow two or even three days at Auschwitz. It is the largest cemetary in the world without a single tombstone. From my readings, the size and gravity of the sight is unlike any other place on earth.

Please visit this site to get some preliminary information. http://www.remember.org/educate/intro.html

Also see the movie the Grey Zone which depicts the uprising in the camp by the Sonderkommandos.