Hello and welcome to AndroidForums.com. I assume you are wanting AT&T's subsidized pricing for the Galaxy S4. Unfortunately, you would have to wait until your upgrade period to receive that pricing or pay full retail. Some people will sell their phones on ebay or other sites to help finance paying full retail to always have the latest phone.
I am also in the same boat with Verizon. I am wanting the new Motorola Maxx, but I still have another year before I am due for an upgrade. I don't have the money for the full retail price.
You might want to check into the new programs being offered by the carriers which allow early upgrades for a monthly fee.