Notice that the version number jumps from Darwin 1.4 to 5. The two versioning systems have different stories. Initially, Apple established the 1.x designation, in order for Darwin version numbers to correspond to the Mac OS X Server 1.x version numbers. However, the Mac OS Server 1.x saw limited use and that versioning practice was discontinued even before Mac OS X was widely introduced. The Darwin 5.x designations, on the other hand, continue the NEXTSTEP versioning (which left off at NEXTSTEP 3.3, followed by OPENSTEP (which later became Darwin), at 4.2). Since the prepackaged "Darwin 1.x" distribution was actually created based on the Darwin 4 code underlying the Mac OS X, Apple decided to ease confusion and to continue with the version numbers it inherited when it bought NEXTSTEP. The command uname -r in Terminal will show the Darwin version number, and the command uname -v will show the XNU build version string, which includes the Darwin version number.
Apple releases a Darwin installer ISO image after each major Mac OS X release. Minor updates are released as packages that must be installed separately. It is likely that the final binary release was Darwin 8.0.