At our club Asst. Instructors are 2nd Dan and Full Instructors are 3rd Dan. I was promoted to 2nd Dan and Asst. Instructor just before I graduated college, that was almost 4 years ago. And I would have hesitations being instructed by a 4th Dan. Actually I wouldn't have any hesitations learning from a green belt that exicutes a particular throw more efficiently than me.
Bottom line, for me knowledge doesn't come from the amount of stripes on our belt or a patch on our uniform. I'd visit the class a few times, based on how things are run and the enthusiasm level in the room you should be able to tell very quickly if he has a deep knowledge of his/her Art or whether he was promoted by an online program that doesn't require video tests :-)
If you do a little research you won't be let down, generally.
Best of luck -