I bought a 40" 4k TV and immediately noticed the difference.
I too noticed the difference. It definitely isn’t an SD-to-HD jump, but it is beautiful stuff. I will say that I really just love the depth of color on all of these new 4K TVs, with HDR and everything. The higher resolution is nice, but the color on a 4K OLED TV with an HDR Blu-Ray is unreal (or more real, I guess).
I love my current 1080p plasma from Samsung, but I won’t be buying another TV until I can get a 4K OLED screen for a decent price.