I have read some information on Intel Technicial and C4 is some kind of SpeedStep Technology. This C4 is "Deeper Sleep" and C3 is about "Deep Sleep".
I don't know if I get it wrong or not.
Somehow I have heard a fuzzy noise out from motherboard too(DV6000t) when on Battery mode. After contact HP and send My Notebook for them to test, they said the problem is on Motherboard (in every dv6000t in my lot) and need to change(and he say must change!! for free. DOA with new one will be the same problem). After I did DOA and claim for new one it's still the same as he speak. But I still haven't try on C4 Disable. I don't see any connection between them C4 and noise? a bomb?