Storage drive manufacturers measure storage drive capacity using the decimal system (base 10), so 1 gigabyte (GB) is calculated as exactly 1,000,000,000 bytes.
Leopard's operating system used the binary system (base 2) of measurement. In binary, 1 GB is calculated as 1,073,741,824 bytes. This difference explains why your 4 GB storage drive appeared as 3.7 GB, even though the storage drive still has 4 billion bytes, as reported.
In Mac OS X v10.6 Snow Leopard, storage capacity is displayed as per product specifications (base 10). A 200 GB drive show 200 GB capacity (for example, if you select the hard drive's icon and choose Get Info from the Finder's File menu, then look at the Capacity line). This means that, for example, if you upgrade from an earlier version of Mac OS X, your drive may show more capacity than in the earlier Mac OS X version.