Hong Kong has a great airport. We didnt do duty free when we there in April as we were going to two further countries so I can't help there. Have a look at their website. 4 hours will fly by!

We passed though HK at the end of last month. There are lots of shops to wander about.
We didn't buy any ciggies (as we don't smoke) or any booze but I did notice they have some portable DVD players but I didn't notice how much they were sorry.