This side-by-side comparison video of 4K HDR Ultra HD and Blu-ray doesn’t show the true technological difference between the two (because things get lost in translation when it ends up as a video on YouTube) but it does illustrate how there is a visual difference between the two that go beyond just more resolution.
4K TVs can often be disregarded because we the extra resolution can't be appreciated unless the screen is really large (think 70 inches and up). But with 4K allowing for high dynamic range, Ultra HD movies can pack a much better colour punch.
Though the GIF above doesn’t do the UHD HDR justice, the video below does show the increased colour depth and greater range of variance. However, it’d be best to experience the difference in person.