Cannot find full memory of pen drive.

I used one pen drive. It is Trans-stack Pen drive and its memory is 4BG. I normally used every time. I bought it on two years. and used every time in full memory. But now I find 2GB. when i send some file over of 2GB then it was hang. I do not know why it hang this moment. but if I used 2GB memory then i can't face any problem. Why it see this problem and what its solution.
