I disagree with you here in the sense that I don't think that 4E is untested. I get what you're saying, which is that it's untested for a CRPG, but it's been out for some time now and clearly has a player base.
Making a 4E game would allow Atari to leverage the marketing WoTC already has in place for 4E.
At any rate, I don't think that the NWN brand has to be tied to the 3E mechanics. Sure, the engine would have to be rebuilt, but it's not like this doesn't happen internally all the time anyway.
I can see your point regarding confusion of players who have used the old system, but at the same time, if you're using the D&D brand, I think it's important to keep in sync with what that brand is trying to represent. And let's be honest, that's 4E right now.