For sure it's been ground into fifth a few times, however the back of the worn part still appears to be flush with the top of the gear. I think it's OK to re-use. With a new pilot bearing and new input shaft bearings and decent ball functioning properly it should stay in gear.
How do the teeth on fourth look? Will you even need road gear that much? How does the other half look?
The tricky part about buying a used one through the mail is that unless you get to see close up photos of the exact part there's a 50/50 chance the "new" one won't be any better than what you already have. What constitutes a good used part can be rather subjective.
If 5th is seldom used and the teeth on 4th are OK then I would use it.