Good stuff Eric!

Yes 4th gear is the most suitable gear to use on the dyno (not in all cars, but at least in an SW20) because it's the closest to a 1:1 gear ratio, but this is NOT as important as many people seem to think it is.
Using a lower gear will increase the torque at the wheels, BUT it will also reduce the wheel speed proportionally, so the resulting power at the wheels is exactly the same. You may see a small power difference between gears due to changes in efficiency in the transmission, but that's all.
One of the main reasons for using a higher gear on the dyno is so that the run takes longer and you get more data samples. Another reason is to help prevent tyre slip against the rollers.