I have slight doubts if this will make approaches much quieter. One might think that one degree will keep the planes higher and enable steeper descent with less power without any further implications, but in fact there are several.
- In my company, it is a requirement at the stabilization gate to have the engines run under a minimum power (10% Torque on the DH8D) to shorten the spool-up in case of need. So the aircraft will need to be stabilized at the approach speed, not still reducing towards it, at 1000ft AAL. Now a steeper approach angle makes speed reduction much harder, usually requiring the early use of flaps, the extension of the landing gear and possibly also increased prop RPM (above 4 degrees, we are even required to have the landing configuration with maximum flaps, prop RPM and the gear down when intercepting the glideslope) - and all this does not exactly reduce the noise emissions. Quite the opposite actually. On the Fokker, a 4 degrees approach may also require the use of the speed brake for the entire approach. While the engines may run at idle power only during the speed reduction, I am under the impression that a few percent of power more or less have a way lower effect on noise than extended flaps or landing gears - these are the other main source of noise on an aircraft.
-Resulting from the previously written, such a 4 degrees approach will on average be flown much slower than a standard 3 degrees ILS. Keeping in mind that aircraft separation on the approach is not measured in minutes but in miles, it seems logical that such a 4 degrees approach will reduce the airport traffic capacity by a noticeable margin. Now this will not lead to less traffic but typically will cause holdings and other delays at rush hours. Consequently, more fuel will be burnt unnecessarily.
So in my eyes, implementing a 4 degrees approach where not mandated by terrain will likely not even bring a small-scale effect in decreasing the noise burden on those living in the approach sector, but will come at the price of increased fuel burn and therefore pollution.