Does the S4 see the card? When you look in storage... Or try to put a photo on the card from gallery... If it worked, just use it. I don't think i ever had to mount one from memory...
You should never have to mount the SD card for it to be used in your device. Now, if you happened to unmount your SD card, you will need to remove the SD card then reinsert the SD card for the system to recognize it.