If you are asking my personal opinion I would say the member with the highest reputation as I don't care about post count. A member can reply "lol" to every post and get a high post count but I value the member who types out one, well thought out reply way more. Organically that person will get more "likes" or "up votes" on his one reply and therefore be ranked way higher. At least that's how I would run my site. In your example every one of those 4 members could have been running around posting junk posts so either way the raw count doesn't tell you much about value.
I appreciate that it means a new way of thinking but it's really a very simple concept to get behind. Every time a user posts content to your site the little counter on their profile goes "+1" ... it's that straightforward and makes perfect sense that it should work that way. It is not something we will be changing. We may consider app-specific post counts or, more broadly, a user stats area under their profile that shows what they are up to Suite-wide. But the simple counter is going to stay a simple counter. It's simple like that. It counts. Counter.
That may be so but I'm sure we can agree it makes more sense to the end user that every post they make increments their count. Sure you might have to adjust your settings a bit but it's only because the system makes more sense.
Be it a post in a forum, a post in a blog comment, a post in a file review, or whatever else it is: these are all still a post that the user made. So obviously that total count of posts shows under their profile. If anything one could say the 3.x way was wrong and should have been addressed long ago. This seems to make basic sense to me. As a user, if I click "Submit" on an input form I don't really think about where that form is. All I know is I submitted a post and I want credit for that on my profile's counter.