While Blu-ray is an improvement over DVD, it is no where near the improvement people got from going from VHS to DVD. 4K has the disadvantage of people needing very large screens to even notice a difference.
I..disagree. DVD to BD was the same leap from VHS to DVD. if not greater for me.
I think it's important to consider that with VHS to DVD, there was not a substantial adoption of new display tech, so this might color that perception a bit. Meaning, DVD was the standard many years before LCD or Plasma broke out of the niche market. DVD was great, but still primarily limited by the previous 4 decades of display technology.
Of course, HD breaking in was the real killer. This is also something that didn't become mainstream until BD/HDVD. Probably not until HDDVD was dead and buried, to be honest.
So, when I think of those transitions: you're looking at most people moving from VHS to DVD on their old 37" 480i or whatever Magnavox sets from Walmart. By the time BD hits the market, people are moving from DVD to BD on their 780p or 1080p Plasma/LCD...or even projection screens.
In a way, it's very hard to compare that transition, because you really have another part of the technology factoring in to perception. DVD added a crispness to the image and removed the obnoxious tracking lines you get with magnetic tape, and of course additional audio options. It didn't do all that much for color depth, in comparison to VHS on the same display.
On HD displays, BD did all of that and much more (especially color depth and contrast, and of course 3x+ ther resolution) compared to what DVD was capable of on the same display.