Welcome to Android Central! You could just move the card from the S4 to the S6, but that would also bring over other data that is associated with other apps on the S4. It might be cleaner for you to copy the photos onto another SD card. The most reliable way would be to unmount the card (in Settings>Storage), remove it, then insert it into your computer. Use Windows Explorer to copy all of the photos to the hard drive (which you should be doing on a regular basis anyway as a backup). Then insert a new SD card, and copy the photos there, then insert the new SD card into the S6.