Could be a few things. A few months back, I used an aftermarket drain plug, that was a hair longer than OEM. It just was touching the shift drum, and wouldn't let me go past 3rd..
Thats a quick check. Back it out , a 1\4 turn and see if you can shift thru the gears.
If not, you'll need to remove the clutch cover and start inspecting around the shift star and the detent spring