4 nights would allow you see many parts of HK (of course the longer you stay the better ). It really depends on what type of travel style you have and what sights you would like to see. I would really recommend to visit Lantau Island (big Buddha)! It's a nice gondola ride and walk when you get to the mountain for nice views. There's lots of hiking spots in HK as well. Make sure to save your stomach for all the food you will be eating!