Fourth Mizukage was obviously controlled before the bijuu was sealed inside of him. This is pretty obvious based on the time-line, Madara was already controlling Kiri before Yagura became a jinchuuriki, even before Rin became a jinchuuriki as she only became one BECAUSE Madara controlled Kiri nin to kidnap her. Yagura was not a perfect jin by mastery/friendship unlike Bee and Naruto, he was a perfect jin because both he and the Sanbi were under sharingan genjutsu even before they were united. We've also seen in the war that jinchuuriki can use their power to the exact same extent as a perfect jin when they are being controlled, coz Tobi's paths were using their powers at perfect jinchuuriki level despite never having such mastery when they were alive. Sure it hasn't been expressly stated that this is the case, but it honestly doesn't need to, Kishi has put up enough evidence in the manga regarding this issue.
