The quality of 4K video streaming and 4K Blu-ray are both high-quality formats, but they have some differences. 4K streaming offers a convenient way of enjoying movies, but the quality is not on par with 4K Blu-ray discs. 4K streams may offer decent resolution, but they often lack the depth and vibrancy of physical discs.
In terms of resolution, 4K UHD is twice that of Blu-ray, providing a better watching experience with greater clarity and more details, making it closer to the experience of watching a movie in a theater. However, Blu-ray discs have a higher bitrate than a 4K compressed video stream, which means they can use more data to represent each frame, resulting in a clearer and more detailed image than a highly compressed 4K video stream. This extra data allows for a wider range of colors, more detail in the shadows and highlights, and more nuanced shading, especially in the dark.
In conclusion, if you care about getting the best picture quality possible, then yes, Blu-ray will almost always be the best version of the movie. However, it's worth pointing out that for most people and a lot of shows, streaming 4K video is perfectly fine.