$VersionNumber

$VersionNumber is a real number which gives the current Mathematica kernel version number, and increases in successive versions.

To find out if you are running under Version 4 or above, you can use the test TrueQ[$VersionNumber >= 4.0].

A version with a particular number is typically derived from the same source code on all computer systems.

See The Mathematica Book: Section 2.13.5.

See also: $ReleaseNumber.