I watch my DVD collection on my 50 inch 4k television, and you know what? they're okay. It's a 16:9 television, so I get to choose between letterboxes with widescreen releases or pillarboxes with pan n' scan releases, but we all do. I'm not worried about 4k content. Honestly I'd be fine with a 1080p panel there.

A computer monitor is a different story; you typically sit a lot closer to it, so pixel density is more significant. I can genuinely tell a difference between my 27" 1080p monitor and my 34" 1440p ultrawide monitor. The latter takes up more of my visual field while retaining about the same PPI. I still don't think I need 4k here though.