mk. let me show you.
on the LEFT, is a used hardened 4th gear hub (03 models +).
on the RIGHT is a pre 03 hub which is NOT hardened and as you can see, the splines are worn down. this is what has happened with you most likely. it slips in 4th when you let off because there is no pressure exerted on it and it can catch its grooves. when under pressure, it won't go in because the engagement teeth are worn down as you see in the pic.
read up more on it maybe you can find another probable cause..
Triple Edge Performance - Home Page
good luck
