juliebuffy - you won't be able to get it back to a 4gb unit because there was never 4gb in the first place. It seems you have received a "hacked" mp4 player, where you and your computer are deceived into thinking there is more memory in the player than there really is. Whenever you loaded more files onto the player than there was memory, they corrupted. Formatting using disk utilities removed the hack and showed the real memory, which is where you are now. I would suggest reading
here and
here for more info about the hack, and contacting your seller demanding a partial refund.