Typically, when you have an SD card in and open the camera the first time it'll ask you if you want to use the SD card as the default storage location. If that's already been done and you want to move existing photos you'll want to go to the 'My Files' app (or whatever file manager you prefer) and go to the DCIM folder locate your photos and then copy/move them to the DCIM folder on your SD card (it should already be there). Granted, I no longer have the S4 but I believe this is how you'll do it based on my recollection.