I saw someone earlier today that had their PhII X2 550 at 1.6V, which is pretty insane but he hit 4.0 that way.
It depends on if you're discussing a one shot or 24-7 operation. I'd say for 24/7 max I would do is 1.45V. For a one shot I've seen the folks here at tom's go as high as 1.55V to achieve 4.0.
But its your cpu, so you'll have to decide what you feel comfortable with. Also, I think it would be heatsink dependent. Gotta keep those thermals under control. If temps look good at 1.45V, maybe it would be ok.
I haven't had my X4 940 above 1.45V.