Let's take a look at the numbers:

PS4 - 143k

Xbox One- 93k

BF week gap: 50k PS4

But that gap is not nearly as interesting as the following facts:

Last year X1 sold 134k during UK BF week, significantly down YOY, in one of its two strongest markets, despite its hyped up 4k blu-ray capabilities and the many great deals.

PS4 has destroyed all the ground X1 gained over the last few months on Black Friday: October gap + September gap = 26k X1

It's reasonable to conclude that 4k-bluray isn't a deciding factor for most consumers. Not now, and definitely not when streaming is the indisputable king.

Well, if you ever needed some numerical evidence of what (IMO) is obvious, then this would be it

Im an avid blu ray fan but the current selection of 4K UHD blu ray movies are not impressive at the moment. I know it is still early days but I do not see the vast majority of existing 1080P blu rays being restored/converted into 2160P. I still wish the PS4 Pro supported UHD blu ray though, it would have pushed me to buy one sooner although I have not seen a Pro running on a 4K TV(I think that would make me buy one).

we own zero UHD blu rays.... soooo.... :p

4k Blu Ray hasn't taken off yet, it's certainly not mainstream. So no it's not a big deal atm. As streaming likely becomes even more popular it may not take off that much at all.

4k Blu Ray isn't important at the moment, and may never be for the majority.

Strangely enough Sony might have just inadvertently killed off any chance UHD blu ray had in achieving a decent adoption rate. Had the Pro included an UHD blu ray player we would have probably seen a surge in UHD blu ray sales.

Well it would have been better but I don't think it would be a game-changer. I think physical media is just on the way out and UHD Blurays are far too expensive at the moment.

4K blu-ray sounds nice - until you see the selection and price point of 4K movies.

Nearly $40 per movie doesn't entice me much. I'll stick with regular blu-ray thx.

I think Sony knew what they were doing.  They have PlayStation Vue to "replace" UHD Blu-Ray if needed.

I didn't buy one so yes, it's completely redundant and useless.
I also don't see anything wrong in this reasoning.