Well, NX could be 4K capable, but only for media content like Netflix and things like that. All they need is a GPU that supports H.265 and an HDMI 2.0 output (because DisplayPort 1.2 is rare in TVs).
The Wii cant reproduce DVD movies, the Wii U cant reproduce DVD movies nor Blu-Ray movies.
I wouldnt hold my breath for disc movie support for the NX bro.
The Wii can play DVD movies with the homebrew hack, so the hardware is capable. With Blu-Ray and the Wii U hardware it might also be capable, but it would need a lot more work to create what is needed on the software side since Blu-Ray is more complex and secure than DVD.
Who knows what disc formats the NX will support, though Netflix is a given.