I'm going to location where my 3G will be pretty expensive. Can I connect my iPhone 4 to WIFI in hotel and share it for my laptop thru WIFI as well? That way I could save some money to not have to buy 2 x WIFI from HOTEL.
No. The iPhone can either act as a client or an Access Point, not both simultaneously.
If the WiFi access isn't registered against the device's MAC address then you should be able to use the login on either the iPhone or Laptop (but probably not at the same time).
If it is MAC address based registration then you could register with your iPhone and then change your Laptop's MAC address to match the iPhone's MAC address so that you only have to buy one WiFi access card.
You're probably much easier off the other way round: make sure your laptop has wifi, and enable internet sharing on your laptop, so that your iphone connects through your laptop to the internet.
I think this will require you to have an extra wifi card plugged into (or connected to) your laptop though (the build-in wifi would connect to the internet, the second wifi adapter would share its connection to other devices, like your iphone)