Dean, I learned something about TH feeders thanks to esshup.. the amount of feed thrown has a LOT to do with how feeder is set up, so your 4 sec rate might be half of my 4 sec rate. I would increase to 6-7 sec if they are cleaning it up in less than a minute.
I messed with my feed cup adjustment to allow a multi-pellet size use and found a small 1/16" change made it so I could unload about 5lbs of feed in a couple of seconds. I guess I'm suggesting you increase time first, you have plenty of room to go WAY up yet.