I've climbed some very steep hills with my SDL, and I know that some of them, I had the car at full-throttle to get up the mountain at 40mph, and the temp guage was rapidly rising. This mountain trek was only about 1/2 mile, but at the top I was at just under 100c.

When you have the car at full load for 4 miles, I would expect you to get an overheat condition like you said.