I have never heard of this. Do you know of any references for this?
This is quite the thread bump! 4" sounds new to me, but I'm only familiar with my own building which is 1.5" concrete poured on the wood. Either way, because the concrete isn't structural (and it's often something like Gyp-Crete which is softer and lighter than structural concrete). It's also poured on the wood frame which which really negates much of the 'soundproofing' aspects of structural concrete as the whole structure is free to vibrate quite a bit more. It's mostly used for fire resistance between floors.
Our building is wood framed with concrete topped floors.
Regularly hear footfalls from the resident one floor up.Other than that, no big issue.
I hear the whole gamut from footfalls to cups on the counter, to chopping vegetables in the kitchen to peeing in the toilet to snoring to an oscillating tower fan in the bedroom. Can't have a dog though in case it makes noise, go figure. The girl upstairs who weights half as much as me makes twice the noise I do when walking.
Concrete topped floors really need to be used with multiple other sound mitigating building techniques to have much of an impact on noise transmission.