If I recall correctly, the rolling stock remained the same, tracks were re-railed and the 4mm came from tolerances that were historically very loose. So the stated goal was to get higher speed and stability from tighter tolerances.

I also remember reading a long time ago that there were two engineering schools: one modeled that tighter tolerances would decrease oscillations and vibrations and the other predicted exactly the opposite. I think in the end they settled on natural experiments. Hope I didn't make this up, need to search for sources.