Just make sure that you log in to Origin with the same account as you used to play Battlefield 4 before. Head over to your "My Games" library and check if you are able to locate the BF4 icon. You can also check the "show: all games" and "show: hidden games" option for the case that Origin is only showing installed games. You should be able to download and install Battlefield 4 from your Origin library without redeeming the code a second time.
Cheers
Basti
