4th gear grinds when changing gear quickly

[Johnmiles2121]

Hey I have recently bought a Subaru Impreza GX Sport Wagon and sometimes when I put it in 4th gear it grinds while driving, I am just wondering if this could be the gear box failing or does the car need more transmission fluid? I always check that my foot is fully on the clutch when changing gear. 

Any help is much appreciated, 

Thanks.