I havn't read the manga but I'm going to throw this out there.

I think that it is very possible that the 4th was around when kakashi became an ANBU. Think about it if he became a Chuunin at the age of 6 (if I'm correct) then how much longer would it have taken him to become an ANBU. Matbe he isn't 17 maybe he's 11 to 13. who knows tell what you think of that.