I have found it depends on the 172 you are flying! There are some 172s I woulod not attempt anything more than 3 people. Get to know the aircraft you are flying befor eyou go loading it up fully, also keep in mind there are variants of the 172 including the 172SP which can climb like a lovesick angel if maintained properly.
It also depends on where you are using it from, i.e. high density altitudes, etc, a 172 that could have climbed fine before may now behave like a brick.