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.