If I get Premium for the Xbox One will I ever have to buy Premium again for my Xbox One account? And if I get it for BF4 will it transfer to Hardline?
Battlefield 4 Premium is a permanent purchase. You won't need to buy it again for this game. Ever.
Battlefield 4 Premium is not linked with Hardline in any way.
BF3 Premium did not transfer to BF4 either.
More information about BF4 Premium is available on the official website.