Anecdotal sure, but I still don't hear the average users thinking of 4k around here. I'd imagine that they are far less than many seem to think in here. Someone probably have TV sales/stats somewhere... Also, I thought most 4k TV sets still got lots of lag for gaming.
The gains they will get aren't worth the risk of waiting just 1-2 years and doing a proper PS5 especially with new PC hardware being around the corner. In my opinion, the transition should be similar to PS1 > PS2 when it was 5 years or PS2 > PS3, but at 5 years instead of 6. The PS2 sold 55+ more million consoles while PS3 was on shelves. Granted, the numbers won't be the same again, but you get the idea.
It's not a bad idea in itself, but these things don't exist in a vacuum. We are getting HBM2 and new AMD CPUs/APUs soon in 2017-2018. Then, shoehorning 4k push with a PS4.5 doesn't seem good enough to me since it will undermine a big selling point of the PS5, which they can just do and release in 1-2 years. I'm sure you will get a lot of hardcore players mad about it as they will have to purchase a new console a wee bit too soon.
Personally, if it means better VR, I'll probably buy it. I plan on making a new PC with the upcoming hardware next year for PC VR and using PS VR to hold me over until then. I will get the exclusives and experiences on both sides next year this way at least.