Here's the real explaination (from me)...
SD Cards are only SD Cards up till the 2GB barrier - the SD Specifications started of specifiying up to 1GB - and then there was a little modification for 2GB. 4GB cards are currently out of standard - unless they are SDHC cards - not SD Cards. The 4GB cards MUST use FAT32 - as FAT filesystem cannot cater for drives >2GB. The 4GB cards also have MASS compatibility ISSUES and everyone has been warned to stay away from them, especially when using devices that are also SDHC compliant, as it can confuse them.
Many card readers misrecognise the size of the cards and have problems, however, I believe you have a dud card more than anything else. A while back, a crop of fake Sandisk SD Cards was released onto the market as 1Gb cards - everyone had issues. Some cards wrote up to 540MB and stopped, others to 882MB ... etc.
Those cards were fake, and had issues which affected the internal memory and controller. They were probably not very compatible either.
- Don't ever deal with 4GB SD Cards, they are out of standards and can cause you problems. They won't be formatted in cameras/devices not supporting FAT32. SDHC cards are the only "SD" cards 4GB or greater that I would buy, HOWEVER THEY ARE NOT COMPATIBLE WITH SD DEVICES, only with SDHC!
- Try a different card reader/writer.
- Get a refund.