The differance in velocity would probably be around 100-150fps i would think.
Depending on the barrel diameter at the muzzle the weight could be as much as a pound but with atypical sporter weight barrel I'd say maybe 4-8 oz and the 4" of length realy woulden't make that much differance in the way that i hunt and I do carry my rifles a good bit , if I were hunting out of shoot houses mostly or realy tight areas I might think about a shorter barrel but for the most part I like them as long as reasonably functionable.
I'd suggest having it built with a 26-28" barrel and see how that works for you if its to much you can always cut it down but if you start off short then your pretty much stuck with it.