bigbadjim, welcome to the Rockbox forums. I've renamed your thread with a descriptive title, as required by our posting guidelines.
Rockbox supports 4GB SD cards. However, Rockbox does not yet have its own native USB mode, which means that for the time being, you need to use the Sansa firmware's USB mode. Since the Sansa firmware does not support cards over 2 GB, it will not recognize the 4 GB card, and you will need to use an external card reader/writer to put music onto the card. Once Rockbox's native USB mode is implemented, this workaround will no longer be necessary.