If you reformatted the 4GB card using the N800 file manager application, the card has been formatted using the FAT16 filesystem and this filesystem only supports a maximum 2GB partition. You need to reformat the card on a PC using the FAT32 filesystem in order for the full 4GB capacity to become available to the N800. The N800 does support FAT32 it's just that the file manager application is programmed to only format cards using FAT16.

Also, as it's a Transcend try using the Serial check page from this thread to ensure you have an original card (you may need to find the equivalent page on the Transcend German site as your signature suggests you are German!)