Could be a fake 4gb drive, although that is a bit odd that someone would bother to fake such a small size. Has it ever shown as 4gb?
Use the diskpart command at a DOS prompt to list the disks, then select that USB disk and do a clean command on it. That would wipe any partitions and formatting on it. If it's really a 4gb drive then it will let you create a new volume and format it to the 4gb capacity.