Yeah, what RockSurfer said...
5th gear is basically for coasting or cruise control on flat highways. Feel no fear at dropping it into 4th on the highway if you're gong up a hill. 3500-4000prm won't possibly hurt these engines, even for extended periods of time. Otherwise you'll eventually wear the 5th gear out. Doubly so if you're running big tires and other mods that increase drivetrain friction.
As for dealing with it, I would of course change the oil - it may be as simple as burnt or old oil. If the noise bothers you, of course run it in 4th on the highway. That's one advantage of manual transmissions - if one gear is going bad not using it tends to keep it from getting worse for quite a while.
And, no, don't believe that nonsense about changing gear oil making it worse - if changing the oil makes it die, it was already damaged.
P.S. IME, changing gear oil every year or two of hard driving is a good idea. The amount of dust and grime that it soaks up is astounding.
