Hard drive manufacturers use bytes to sell their drives. In reality, GB is the measurement. To have 4GB, you need 4,294,967,296 bytes.
The drive has 4,000,000,000 bytes or 3.72529GB.
1024 bytes = 1kb
1024 kb = 1mb
1024 mb = 1gb
1024 X 1024 X 1024 = GB
So 4,000,000,000/1024/1024/1024 = 3.72529GB