A 4K Blu-ray has a much higher bitrate (up to 100 Mbps) compared to a WEB-DL (usually 15–30 Mbps). This means the Blu-ray has more "data" per second, leading to fewer artifacts in fast-moving scenes.
Because it is not re-encoded by the person who "ripped" it, the file retains the exact bitrate and color depth (often 10-bit HDR ) delivered by the service provider. WEB-DL 2160 vs. Other Formats