Basically no disk I've encountered so far has the exact amount of space available that they write on the box. This has a combination of factors besides trying to sound bigger.
In your case (disclaimer: probably), 4GB in bytes that is multiplied in 1000's instead of 1024's is 4000000000 bytes, dividing that by 1024's results in ~3,72GB.
The remaining 0,04GB (~40,96MB) is then reserved for the filesystem (which remembers the filenames and physical locations on the disk and stuff).
So yeah, there isn't a way to make it bigger as it's already as big as it physically is.