For 4K, you're going to need a 3080 or a 6900XT. You could get by with maybe a 3070 or 6800XT, but 4K is pushing it a little on those.
As for monitor, 4K gaming monitors are a very nascent category, and I'm not familiar enough with the options to recommend one in particular. However, they're all like $1500 or more, and feature packed, so probably any of them are great.
There's more pedestrian 4K 60Hz monitors that are more designed for productivity (photo/video editing). Those are more reasonably priced and decent in their own right, but they're not designed for gaming.