you could use a 3.5 " ( 12inch OL ) or 5.5 " (14 inch OL ) on a gen 4 without changing overall outside width as long as the outer bell is not more then 8.5 inch
I don't encourage it but You could use a 2 ' inch but if you go 12 " OL it will add to the outside width, that applys to Gen 3 or Gen 4