A 4K stream and an Ultra HD Blu-ray of the same film can deliver a 3,840 x 2,160 pixel image on the same screen. However, that doesn't mean the television receives the same amount of image information. Resolution describes the grid of pixels on screen, whilst compression determines how much detail is preserved within that grid.
For conventional streaming, that distinction gives Ultra HD Blu-ray a built-in advantage. A disc can dedicate far more data to video, whilst an internet stream must also maintain smooth playback as network conditions change. This is where data rate matters, typically measured in megabits per second (Mbps). However, data rate is not a universal score of quality. The codec also matters. A codec is the technology used to compress video for storage or transmission and then decode it for playback. Some codecs can preserve more image detail than others whilst using the same amount of data.
According to the Blu-ray Disc Association, Ultra HD Blu-ray supports maximum video speeds of 108 Mbps on 66GB discs and 128 Mbps on 100GB discs. Those are maximum limits rather than minimum or typical data speeds for films, but they show how much data the format can dedicate to video. By contrast, Netflix recommends at least a 15 Mbps connection for 4K, though that is an internet speed recommendation rather than the actual data rate of a Netflix video, and it can reduce image quality on a slow or unstable connection.
Not all scenes are equally easy to compress. Large, relatively simple areas can be represented efficiently, whilst fine textures, rapid movement and details that constantly change need more data. Film grain is especially demanding because its seemingly random pattern changes from frame to frame. If there isn't enough data available for a complicated scene, those compression artifacts become easier to detect.
Some services stream 4K at much higher data rates. Sony Pictures Core's Pure Stream operates between 43 Mbps and 80 Mbps, with the streaming service requiring at least a 115 Mbps internet connection to reach the highest setting. Sony claims that top tier delivers an experience comparable to 4K UHD Blu-ray. That clarifies the distinction: streaming itself is not necessarily the limitation. If an internet stream has sufficient data rate and bandwidth, it can come much closer to disc quality.
