Macbook Air SDHC card problems

I recently got a mid-2011 11" Macbook Air. It is running on Mac OS X Lion 10.7.4. This computer does not have a built in sd card reader. I use a USB card reader to attach my SD cards. I am able to use a 2GB SD card, but when I use a 4GB SD card, it does not recognize it. Neither iPhoto nor the system recognizes it