Acceleration is the key. The very best breakin procecure will involve acceleration. You simply need to keep the engine working. This is what the owners manual is refering to when they say, vary your speed, and do not drive at a continous speed. By keeping the engine under somewhat of a load, you are keeping the rings pushed out in a perfect circle. No long cruising in the upper gears. This is so important on an engine. You don't have long to do this, so play around by driving the car in a somewhat wastefull manner. I really think the 4000 rpm is a bit conservative, but that may be done for the same reason the speed limits are set back a bit from what they expect you to run. Stay off the red line, for about 500 miles, but 4500 would really be fine, IMO