Originally Posted by ZoomZ

My background is Aviation. Air cooled, Piston, N/A aircraft engines are completely different beasts. They are nothing more than large lawnmower engines. Relatively speaking.

Having said that, they are still engines with pistons, rings, valves, camshafts and bearings

To put it simply without too much technical explanation:

avoid travelling at a single speed or RPM for prolonged periods since varying engine speeds are vital to proper break-in as they expose internal parts to a wider range of heat, pressures and stress. (notably the piston rings.)

In todays cars, I'd narrow down to the two more important important terms: Pressure and stress. Heat should not be an issue in a proper Water cooled system. (unless of course you decide to break-in really hot weather.

That brings me to the next argument. The 4000 RPM for first 1200 miles is Determined by Nissan to cover a wide range of users and climates and scenarios

It assures proper break-in, without overheating or excess wearing if conditions become extreme.