There is an option in the Asciidoctor PDF Theme:
https://github.com/asciidoctor/asciidoctor-pdf/blob/master/docs/theming-guide.adoc#page-numberingYou can add 'page-numbering-start-at: 5' to your theme to start with page number 1 on the physical page number 5.
When your preface or dedication grows or shrinks, you need to adjust that number manually.