Go with the biggest port that makes the tuning you want achievable. The full length 4" might not end up fitting. To be honest a small port like that is easy to make, i would just make both and find out. I would guess the 3" will have more range and 4" would be a little louder closer to the box tuning. Gotta remember every situation will differ, no way to know what will work "best" until you try it. Good luck n hope i sorta helped.