As with any conference - people come, to a great extent, to be entertained. The most memorable talks (other than the horrible failures) are the ones that are out of the ordinary: with cool examples, with humour.

You have to be very, very lenient with your audience - 4 hours is long, so don't expect to give four times the amount you'd put in a one-hour-high-octane lecture. People will simply tune out. I have a tendency to cram way too much information in my talks, so I always try to tone it down. This is even more the case with long lectures.

Good luck though, this sounds very cool.