While it is true that "warm air rises to the top" in confined spaces with very little air flow, that is NOT what happens in a computer case with four fans pushing air around! The important factor to consider is to maximize air flow, and that usually is done best by balancing intake and outflow, and by arranging a clear flow path. Many people arrange air flow from front (intake) to back (exhaust), and / or bottom (intake) to top (exhaust).
OP, how are you controlling the fans? I expect the one in the PSU is not adjustable, so you have 3 that you can control. Are they connected to mobo headers using automatic control, or have you done something else that might limit their performance? I hope you are not using "Low-Noise adapters".