I’ve streamed 1080p reliably over 4G LTE on T-Mobile and also tried TruPhone (they use the T-mo network but are way cheaper) for wireless data but make sure you’re truly uncapped for data or the bill will murder you
Didn’t know the iPad of any sort supported 4K viewing. 🤔
Anyway, to view 4K iirc, an average of 30Mbps is needed. Don’t know of any carrier that can reliably provide that unless you live under a tower.