There is no such thing as an 8GB (or 4, 16,32, etc) SD card, they are all
SDHC is FAT32 formatted, SD is FAT16. Since FAT16 has a 2GB partition size
limit and you can't have multiple partitions on an SD card even if you did
manage to format a 4GB or higher card as FAT16 it would still only use 2GB
of the card.
Beyond the format type there are other odd limits too, a friend has a laptop
that will read a 1GB but not a 2GB and I know of an old HP digital camera
that corrupts the pictures if higher than a 256MB card is used.