Crap i need to rephrase, the 4th was Jirayas student, but the 4th was Kasashi's sensei, I think. I took a look at Kakashi's age and it wouldn't make much sense so my apologies, so in correction I still think Obito. Oh and the last bit is that, whomever Kakashi received the eye from it was a high level Uchiha like Itachi because his eye contains three dots.