I have the schematics here. Check page 6.
http://hamradio.lakki.iki.fi/new/Doc...Schematics.pdf

I originally was looking at this when I was researching the USB charger mod and I could of sworn there was a regulator on that line before it made it to the charging IC, but I can't find it now. 4V is the operating voltage. 6V from the charger is the typical charging voltage.

What I'm thinking right now is that the nokia provided charger is smarter than I originally thought. To step down the voltage stably enough from a range of 100 - 240 fed to the lithum. I just don't know without cracking open the thing.