Factor in the post production and duties afterward, as well. How long will it take you to edit it? How do they want it delivered to them? (For example, if they want a polished DVD w/ menus and whatnot, that would cost more.)
That's when the hourly rate could run into a dilemma, unless you're not charging for post. Because a 4-5 hour filming could have 4-5 hours of editing afterward, easily.
I've done a lot of independent web design for local businesses in my area, and I've always had a good experience with charging a flat rate. Obviously it's not the same as shooting a video, but the reasoning is still similar. I've always felt that hourly rates can easily be used to take advantage of uninformed customers, so I've liked to charge a flat fee and I usually get half of it before the project, half after it's done to their satisfaction.
I'm a minority, though. I probably make less money than charging an hourly rate, and I certainly give people the opportunity to overwork me to their satisfaction, way past whatever I charged, sometimes. But, since I'm a student, I get more business by charging the flat rate. It's a double edged sword.