What is the purpose of the resistor on this MOSFET's drain?
Given that you have said that the output line goes to an array of LEDs, I think that R4 is there to quickly discharge the LED capacitance when the FET is turned OFF.  This would minimize ghosting if the LED array is multiplexed quickly.

I used to run into that problem with some of the large LED arrays we used to build many years ago.  Adding a low-value resistor to quickly discharge the charge stored in the LEDs reduced or eliminated the ghosting problem.