you should already have one in the cam.

You would've  gotten a  message  on screen  if  you didn't have one in the camera.

The one  I  had in mine was a little small,    so I got  a  32gb    card.     32 is  about as big  as  the   system  can handle without  slowing  down.    

Mine was  the  cgo2+,   but only had  a  8gb  card.   Yours  may be larger because of the  4k potential.

Don't forget to  format  the  new card too.     FAT32 is  preferred  format.