I have the KitchenAid 600 mixer also. It was heating up just mixing two classic loaves at a time. I found that by splitting the oil and honey between the oatmeal and the flour mixture (half the oil and honey in with the oatmeal and the remaining half of the oil and honey in the mixer bowl with the preferment and flour) it loosens up the dough enough for the mixer to fairly easily handles 3 loaves. It also makes mixing in the oatmeal easier. Letting the dough sit for a few minutes before adding the oatmeal also seems to help. The bowl seems full with three loaves and 3 bake better in my oven so I haven’t tried four.
I’m sorry, but I just don’t think they can handle 4 loaves of our big, heavy dough.  I learned that lesson the hard and expensive  way.  Maybe just make 2-3 loaves at a time.  This project welcomes all donations, small or large!
Recently picked up a used KA 600 series mixer. Definitely struggled with a four loaf formula. Couldn’t get through the full mix without shutting down due to overheating. Had to finish the batch by hand kneading.
I know these machines need some rest during the knead. Anyone with one of these that has some wisdom to share to get more out of it?