The Canon Rebel/300D LCD display cannot display a shot count # greater than 999. This is not an error with the camera, the LCD simply does not have a fourth digit in the shot # display. Just use the camera normally with the 4Gb card. The number display will start working again when the amount of free space on the card drops to a level where it will not hold more than 999 more pictures.
Eric Pohl
Camera Repair Technician
United Camera Repair