The Q9650 is an unlocked multiplier CPU, so if you have the proper cooling for the CPU, 4ghz may very well be possible but you'll just have to go for it and see, there are no guarantees.
Sorry my bad it was the QX9650 that had the unlocked multiplier, but you can still overclock the FSB and with the proper cooling should get at least to 3.8ghz and maybe possibly reach 4ghz?
There were more than 1 model of G41 chip set motherboards released by ASUS so the exact model # of the motherboard you have would help answering your question?
http://www.tomshardware.com/forum/259899-29-core-overclocking-guide