There isn't a Netflix 4k App as such, it's the Netflix subscription that's 4k or not. The app you've installed is a mobile version, hence it looking SD/stretched/just generally crap. It's not made for TV's.
A lot of owners of formuler devices recommend them to people if they just want something for watching iptv. If you've got a formuler device and want to access other streaming services such as Prime, Netflix, iplayer, DAZN, Disney Plus, ITV X, My5, Peacock, Paramount+, Hulu etc you're probably better off putting them on a firestick or smart TV to run alongside your formuler.