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.