I have an HP Pavillion G4 laptop

You've not said where you are travelling from, but I'll assume it's the United States, since for almost anywhere else in the world this wouldn't matter.

In the US the power is delivered at 110V, 60Hz, whereas in the UK, Europe and most of the rest of the world it is 220-240V, 50Hz. If you connect a power supply for one into a mains outlet for the other you may be lucky and nothing will happen, or you may be unlucky and your power supply may fail.

However, most power supplies for portable devices are now designed to be compatible with both systems: all you need is an adapter to be able to physically plug it in.

On your laptop's power supply there will be a label such as this:

Look for the line resembling the area I've highlighted in red.

On this example the INPUT is listed as 100-240V, 50-60Hz. This means that this power supply will work on the US 110V mains, and on the UK 240V mains. If your power supply has this printed on the label you'll be fine. If not you might need a voltage adapter.