I thought I posted here ?
To hit 4Ghz on a Q9450 you'd have to run a 500FSB... That's quite high and I don't think all the cores in a Q9450 will scale that well.
The Q6600 G0 is built far better in that way, you can always get up to 3.5Ghz without much troubles. And 4Ghz is easily done with good cooling and an Asus board or something that overclocks good as well. You only need to run a 445FSB for 4Ghz on a Q6600 (9 multiplier).
So it'll clock higher easily and costs over a hundred dollars less... I've seen it get up to 4.2Ghz even.. on WC