The Codex examples for next_post_link infer that you can both restrict the link to the current category by making the third parameter 'true' and exclude categories with the fourth parameter.
This is not working for me, however. I can do one or the other, but not both.
I am wondering if the function broke somewhere along the way as it seems to have worked at one time.
For what it's worth, I am not the only one who seems to suspect that this is the case.