I think it's also the scale or size itself that is somehow more attractive. When you see something that small with so much detail packed into it, it is more...I don't know, impressive to me. Just seems much cooler when it's is shrunk down to that scale. I don't know, the 6" line just does not connect with me for whatever reason. I can't explain it. They do look nice...but the appeal isn't there like it is with 3.75".
I think the success of the 4" action figure is purely in the math. Any smaller than 3-4" and the figures become
too small. You lose articulation, detail, playability, all that good stuff. Any larger and they're
too big. Sure you get the same if not more articulation and detail, but size (and price) become an issue. 4" figures are just the right size for vehicles, playsets, and other accessories. And there are a plethora of reasons 4" action figures have been the backbone of the action figure market and so many toy lines for 30+ years now. With the 4" scale comes the ability to develop other tie-in items that diversify the line and add appeal to it as a whole. Overall, it's just optimal.
Even as a kid, whenever I had larger figures, I always gravitated back to 4" figures whenever I played with them. Perfect example were Mighty Morphin' Power Rangers. When they first came out, I got all of the 8" figures and really liked them. They were nice, had loads of articulation and looked great. However, when I wanted to take them to school so my friends and I could play with them, I could never bring more than one or two due to their size (I was only allowed to take whatever could fit in my backpack). But the 4" figures were perfect and I could bring a whole lot more. Same goes for the various toys I had that utilized smaller figures. Their lack of ability to do much made them boring to me, and I always gravitated back to 4" action figures. Anything else were just 'curiosities' really.
This Kylo Ren figure is a perfect example of what great toys Hasbro is capable of producing, especially in this scale.