Why doesnt the 4gb memory card work in your digital camera?

It's likely that your 4GB card is an "SDHC" card, and your camera is an older camera that was designed before the SDHC format came out. Although SDHC cards look exactly like standard SD cards, they are not. If your camera is a few years old, only SD cards of 2GB or less (standard SD cards) will work with your camera.

But hang onto that 4GB card. One day you'll have a newer camera that will make good use of it.