I am not familiar with the Sussex breed but 4lb at 17 weeks is really good for persona use, old enough to develop flavor but not to old that makes it not worthwhile.
My problem with breeding projects is that I never write anything down, I say I will remember and then forget an hour later... hey, it used to work for me year's ago.
Just remember, as kin mera pointed out, pay attention to the leg size, it won't do any good to finally get the weight gains you are looking for if the legs won't support it.unfortunately I spent a year on a project without thinking of that( and I must admit, I just like the look of a Delaware, just not the temperament of them)