Can IPhone 4 share WIFI over WIFI?
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.