For what it's worth, I don't believe that the 4 for $20 is that great of a deal. It makes it an average of $5 a movie, which is decent, but I know I've been burned every time I buy simply because I see the title I bought somewhere really cheap new (BB,CC etc). Right after Nacho Libre came out and was still $15 (2 for $20), I had a coupon making it $7 or $8. Thought that was decent since it was so new. A week later it was $5 at BB. DVD's drop FAST, within a few months they can be had for $8-$10, a few months more and they're $5, and new. If you can find some really cool ones it's okay, but since they are also usually barebones (not special editions) and movies within the last year or so, there's a lot of crap.
If you do go this route, here's a suggestion: go at a slow time and ask the employee if they have any extra, non-locking amaray's (cases). The ones blockbuster uses are really crappy, but they get Amarays from time to time (not as much as when they didn't use the coverart), and they look much better; shinier and just, better somehow. If you ask nicely and tell them you just want your DVD's to look nice, I bet they'll have some for you. Sometimes I just do it for customers without prompting, if they're really cool.