Fourth gear is pretty much the industry standard for doing a dyno run. It makes it a lot easier for people like us to argue numbers on a level playing field. It also gives a little more precise graphing of how your engine runs, due to the longer time it takes to get to redline in a higher gear.

If all dyno runs were done in 1st or 2nd, the run would be over in a couple of seconds. Using a higher gear takes longer, and at least provides a much more accurate HP/Torque curve.

The biggest problem is that you'll get diferrent numbers from day to day, and dyno to dyno.