I have been have problems with my DV4000 charging, it would charge only if I connected a external monitor to it, it was using that circuit as it earth source, you only had to connect it with no power.
It turned out to be the AC adpter socket, there is a tiny micro switch inside the socket which is opened when the adpter is plugged in, in my case the switch was damaged and wasn't switching as it is surposed to.
The laptop was taken apart and the socket unsoldered and replaced with a new one, approx £15.
hope this is useful to you all, thanks also to Colin from : http://www.truestate.com/hprepair/ a really helpful genuine Guy.

Regards Bob

This is a very common problem with laptops in general. People first suspect and replace the AC adapter, then find out that the DC jack (in the US) is bad. Some of the DC jacks are on a separate cable, and some DC jacks are soldered onto the motherboards. I have a Toshiba Satellite with a soldered DC jack. This jack cost $42. The laptop has to be completely disassembled

I Have replaced two other HP laptop jacks, but they were obvious fault's as they had become loose and had intermittent charging and the jacks for these laptops were of different design and didn't have switches inside them, the DV4000 on the other hand showed all the signs of being ok, as mentioned they would charge normally if connected to an external monitor suppling the extra earth.

"...as mentioned they would charge normally if connected to an external monitor suppling the extra earth"

This must be an English thing... I would say " the laptop charged normally if I connected an external monitor, supplying additional grounding"...