Synchrony offers such high yields on savings because their big product is consumer credit cards, and even at 4% APY on savings accounts the spread to a 25% APR credit product, even factoring in risk, makes it extremely profitable to do so.
That's really the big reason why savings rates vary so much between banks, if you have a bunch of consumer credit lines then high savings rates make sense, if you have a bunch of lower fixed-rate collateralized loans then you can't get away with lowering your spread so much.
That's really the big reason why savings rates vary so much between banks, if you have a bunch of consumer credit lines then high savings rates make sense, if you have a bunch of lower fixed-rate collateralized loans then you can't get away with lowering your spread so much.