Are you saying that streaming a movie (say a recent Marvel one) in 4K from Disney Plus will look noticeably less good than if I played the same movie from a Bluray player?
What about a Bluray torrent?
A streaming service would probably compress that same 4K movie down to around 10-15GB tops, but probably even smaller, again depending on the same factors. In my opinion most people probably wouldn't notice a huge difference, if any, but it would definitely be noticeable if you knew what to look for (loss of fine detail in fast-moving scenes with a lot going on, a smooth gradient in a low-light scene on the blu-ray might have noticeable banding on the streamed version, and other compression artifacts like that).
Blu-ray torrents could fall anywhere on the spectrum depending on how it was compressed by whoever is distributing the torrent. If you see one labeled "remux" that usually refers to a video file taken directly from the Blu-ray without any additional compression, so watching that one would give you the same experience as watching the blu-ray directly.
When downloading video, you can grab a remux (the raw video files off of the disk, just stripped of DRM), or a re-encode (those video files compressed again using a better codec). A re-encode will be about the same size as a streaming release and generally has higher quality. Re-encodes tend to use cutting-edge video compression, and pirates are willing to throw more compute time at a single output format than a streaming service which has to produce video files for many target devices.
A 4K Blu-ray (or remux) will get you the best quality, followed by re-encodes, and then web-dl's.
https://preview.redd.it/1l50i8zynts51.png?auto=webp&v=enable...
For a more recent example, I found the desert/spice scenes in Dune to be unwatchable on HBO Max but very pleasant in my subsequent theater visit and later on the 4K disc. In my experience Netflix is the worst offender of the major streaming platforms, and iTunes/Apple TV+ can approach some Blu-rays (though there’s no trick here: they send more data down the pipe).
But also the bitrate alone should tell you something. You will typically get around 15-20 Mbps for 4k streaming, and often lower, though Apple's service apparently will occasionally hit up to 40 Mbps. 4k discs have a bitrate of around 128 Mbps.