If it is a 4GB card, then it can't be an SD card as they have a maximum capacity of 2GB. If it's 4GB then its an SDHC card, a Secure Digital High Capacity card. Your D80 appears to be fully compatible with the SDHC format, but your card readers do not appear to be capable of reading this format of card.
Everything you ever wanted to know about SDHC can be read here:
http://www.sandisk.com/Assets/File/pdf/retail/SDHC1.pdf
Ensure that you have a reader on your PC that can read the SDHC format.
Hope this helps.
Duncan