What is 4K Ultra HD?

4K (Ultra HD) is a screen resolution of 3840×2160 — four times the pixels of Full HD 1080p — producing visibly sharper detail on larger TVs and monitors.

To actually watch in 4K you need every link in the chain to support it: a 4K TV, a 4K-capable streaming plan (e.g. Netflix Premium), a title mastered in 4K, ~15–25 Mbps of bandwidth, and an HDMI input that supports it.