I wanted to buy a 4" cowl induction hood but they are pricey and fiberglass and some peopple told me they worp some times and it's hard to align them. What do you think about getting a scoop like this one and mount it on your existing hood. You still have to paint it but you know the hood fits and it's a LOT cheaper.
114 is the one i like.
http://www.up22.com/scoopw1.htm#SC-114