I agree on the stability issue. I haven't seen a problem with the four-section tripods.
One other thing to consider is maximum height. If you get a 13 series Gitzo (which is what Michael uses in the DVD), the four-section tripod has a higher maximum height than the three-section equivalent, and his comment about flexibility on terrain is absolutely true. If you get an 11 or 12 series Gitzo, the reverse is true. For example, the three-section 1227 has a higher maximum height than the four-section 1228. You need to look carefully at the specs, and try to get something that minimizes the need to use the leg collars.
Finally, think about situations where you might want to travel with the tripod. For example, will you ever travel on an airplane with it? If so, you'll probably want to put it in checked luggage. Make sure you can fit your tripod in the luggage you plan to take. Tripods like the 1228 easily fit in a carry-on, but the 1227 can be a tougher fit.