GoKawiilThis is where bitrate matters. Bitrate measures how much data a video uses over time, typically in megabits per second (Mbps), but it's not a universal quality score. The codec matters too. A codec is the technology used to compress video for storage or transmission and then decode it for playback. Some codecs can preserve more picture detail than others while using the same amount of data. HEVC, for example, was designed to compress video more efficiently for uses including streaming and digital storage.
Even with an efficient codec, the amount of data available still matters. The Blu-ray Disc Association says Ultra HD Blu-ray supports maximum video rates of 108 Mbps on 66GB discs and 128 Mbps on 100GB discs. Those are ceilings rather than minimum or typical movie bitrates, but they show how much data the format can devote to video.
Internet streaming is built around a different set of priorities. Apple's HTTP Live Streaming (HLS), one system used to deliver video over the internet, provides multiple versions of the same video at different bitrates so playback can switch between them as network conditions change. Netflix shows that tradeoff in practice: it recommends at least a 15 Mbps connection for 4K, although that is an internet-speed recommendation rather than the bitrate of a Netflix video, and it may lower picture quality on a slow or unstable connection.