Just like clothes or cars Adult DVDs vary in price. Its hard to sift through all the choices to figure out what is the best value.
Lets start with what the industry calls "Features." Back in the old days a studio shot a movie. Just like in Boogie Nights they shot it like a low budget Hollywood Film and took it very seriously. This film eventually went to adult theaters to be viewed just like any other movie. The VCR changed all that. The studios realized that people were willing to pay big money for a copy of these movies to have in their own homes. Early VHS releases were over $100 each. The studios were now making money twice on the same movie. The studios were not down yet. They said why don't we take all our movies and compile all the scenes with blonde women and release a new movie called "Best of Blondes." Now the studio had a third release of the same footage. They were making money 3 times off the cost of one scene. This was the basics but as cable TV, PPV, and the internet began there were even more opportunities, but this lead to the "comp" or "compilation" DVD that is a standard today.
When you see a movie that doesn't give the amount of hours its usually original footage of 90 to 120 minutes. It will be higher quality.
When you see a movie that says "4 Hours" these are a collection of scenes from older movies with half the resolution of the feature. Most can't tell the difference and if you have never seen the material before then its new to you.
This is the basic history on the "features" and "4 hours" that we see today.