Check if the SD card is iserted properly into the camera, eventually pull it out and push it back inside the camera. Also remove camera batteries , repeat the operation, and put back batteries.
If the card still does not work test the card into a different device, it is not unfrequent that an SD card comes defective from factory. In this case take the card back to the retailer and a have it changed.
If the card works on a different device, then it may be that the NV3 does not support 4 GB , there is no mention about max SD format supported in camera specs
, or in the manual. In this case I suggest testing a 1 or 2 Gb card at the shop, and then buying right one.