Fiat Bravo gearbox

"ok when im in 4th gear you can get what only can be described as a faint sound like woooooooooooooo as you wind through the gear 1st 2nd 3rd and 5th gear is all completley silent and seem fine. im assuming this will just mean there is abit of wear on the 4th gear but if its only just started to do it how long do we wreckon i got till she goes? i am trying to source a performance gearbox or maybe a 6speed box as that extra gear would be usefull i believe. all thoughts welcome "

First thing I would do is change the gearbox oil if you haven't already. Not sure what price a 6 speed costs for just for the box but the supra from 5 speed to 6 is 1500 + fitting.

agreed oil change