4 times a year, which gives an average of 3 months per update. So, we can expect new PR update for every quarter of the year.
I would have said a few years back 'yes' it would have been pretty reliable to say that updates occured 4 times a year but as of late it has not been as reliable so i would start to say 'no' and anyway i dont check these days as its going out of fashion.
I'm not really sure cuz I never look into it but I know for a certain that they do PR way more than 4 times a year
I think Google updates PR daily, might be it was 4 times in early years as you mentioned. Now Google Data center has vast capacity and they can do the updates daily.
As i have found changes in my client sites on these dates and also it was confirmed on the dates mentioned that there has been a PR update.