C172, PA28-161 and PA28-181 have about equal payloads. Of course it depends on the exact model and avionics fit but generally they're about equal. They will take four average adults but you can't fill the tanks all the way in that case.
The two adults in the back have to be madly in love with each other to be able to avoid irritation after two hours anyway, both in the PA28 and in the C172.
If you want the capability to fly with four adults, baggage and full fuel, you need to look for a six-seater.