Amazon offers a large library of 4K content, often at no extra cost beyond the Prime membership. However, finding it can be tricky (look for the "UHD" tag in the top corner of the thumbnail).
Fantasy lives or dies by its texture. In standard HD, armor looks like painted plastic. In 4K HDR, you can see the hammer marks on Geralt's swords and the rust on the armor of the Night’s Watch. House of the Dragon specifically uses very dark lighting for its dragon scenes; 4K HDR allows you to actually see the details in the shadows without raising the black levels. UHD 4k TV-Shows
Netflix produces more UHD 4K TV-shows than anyone, but they aggressively compress files to save bandwidth. Look for their "Netflix Original" tags marked with Ultra HD 4K. Their Dolby Vision implementation is excellent, even if the fine detail sometimes gets crushed. Amazon offers a large library of 4K content,