Perl 4 and the book's first edition were released simultaneously, and Perl 4 was merely a version of the Perl 3 interpreter.

Depending on who you ask, this decision was either for convenience ("this book teaches Perl 4" is easier to say than "this book teaches Perl 3.numbers.here") or marketing ("look--Perl 4 came out!"), the latter sort of like Slackware's increment of version numbers from 4 to 7.