Most common problem is the Card Reader is not SDHC compatible. It will read 2GB or less SD Cards, but 4GB cards are SDHC format not the standard SD format.
But you said you had used the 4gb card with the reader before? Correct?
The reader ususally will have the SDHC logo on it like the one on the Extreme III SD Card if it is SDHC compatible.
Robert