Is this valid?
Forget code for the moment, and examine this, clipped from another site:
4 in. soil pipe offers a more efficient way to dispose of solid material, but the introduction of low-volume toilets and other water-conserving measures, significantly reduces the level (height) of water in the pipe, resulting in significant clogging within the soil pipe. In new construction, the 4-in. pipe is now too wide for long, horizontal runs (especially between the home's main vertical drain, or stack, and the street).
3-in. soil pipes raise the flow level in horizontal runs and, as a result, suffer fewer clogs.
This discussion has been going on since the introduction of low flow toilets in 1992. Even muncipal waste water departments have expressed concern that the flow in the mains might be too low to provide good solid carry.
I have not read any definitive scientific test reports on this matter. I am not aware that any codes have been changed to disallow 4", or that municpal sewer designs have changed.
This is certainly a valid topic for discussion. Please fill us in if you have anything to add or comment on this issue. If you are looking for research, two sources might be the IAPMO website, and PMEngineer magazine on line.
The codes have not changed when 4" is necessary. The issue is the velocity of the fluid through the pipes. The larger the pipe, the lower the velocity for any given volume of water.