This happened to me when I replaced my 2GB card for a 4GB for my sat nav.
You need a different card reader. One that says it will specifically take the 4GB SD card.
I got mine on Ebay-It's called the 'All in one mini card reader'. It seems to read just about every card on the market and cost less than a fiver. They have a USB connector
Geoff in Wales
June 2009