I went out for some MX practice on my '04 YZ 450 yesterday afternoon and was going through the gears on a straight. All was well until I shifted to 4th, and it started hesitating and making a slight grinding noise and felt almost as if the clutch was slipping (definitely not the clutch though because the other gears are fine). I tried 4th gear several times after that, and something is wrong. Every time it hesitates, barely accelerates, and makes the grinding noise. Is 4th gear stripped or worn? If so, is it a pretty serious repair (i.e., splitting the cases)?