That's due to the difference between binary and decimal GB. In decimal, 4GB is 4.0 billion bytes, whereas in binary, 4GB is equal to 4.3 billion bytes.
A binary KB equals 1024 bytes, so 1024 x 1024 x 1024 x 4 equals 4.295 billion, rounded to 4.3. The file size is stated in decimal GB, but the actual file structure is in binary.