the charka the fourth hokage took and sealed from him is permanent. why? because when the fourth took it from him he shrunk in size
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see how much bigger his paw print and how smaller it is now. and he was sealed into naruto like that so the fox lost half of his charka forever. and the charka he takes from naruto is just to stablize that form he had since that day it was sealed. also in this link the fourth has the has the same seal on his stomach like naruto keeping the charka contained. so for a short time minato was a jinchuriki like kin and gin then he died along with the charka he stole from the fox.
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look how skinny he looks when naruto took that charka from him. but over time he ate narutos charka and went back to normal(but still missing half of that charka the foourth sealed)