This guy at the sportsman show told me that no one ever shoots 3 fletch 5" feathers anymore and that I should change with the times.
And he was trying to sell you... what? The guy's full of crap.
4" 4 fletch will give you more fletching surface area than 5" 3 fletch and will give the arrow slightly more stability. However, like Steve said, it will add a tiny bit more weight to the arrow, increase the drag and decrease your arrow speed. Not only that, it will cause more clearance issues with the rest (unless you use a dropaway) and be noisier. I think 4 fletch is okay if you like spending your time and money to get more problems than you started out with.