That's probably not the answer you are expecting, but if you are physically extracting your PS4 HDD you can plug it in a PC and use any utility able to read the SMART informations: you will get a bunch of informations and, among them, the number of hours it has been powered on.
Very unpractical, I agree, but it depends on what purpose you want this information for: if you want to know the wear-level of a console you just bought it's ok, but if you want to track weekly the use of a family shared console, then you will have to look for another solution...