For 4 hr rides, I bring 3 bottles, each loaded with 1 cup of maltodextrin - I buy it bulk at $56/50lbs shipped from Honeyville. Gives 300-400 cals/bottle, and it works great - I'm not even hungry after the ride, and I do mostly big mountain road climbs. For longer rides or hotter days, you will need access to more water than 3 bottles and may have to bring some powder (or food) for the 4+hr mark.

I think "Carbo Gain" or something like that is a big tube for about $2.50/lbs of pure maltodextrin - same as the bulk stuff, just about 10 lbs worth.