Pages applications introduced after Oct 2013 are not based on the iWork '09 or earlier applications code or internal document formats. Many of the features last seen in Pages '09 v4.3 have not been replaced or may be implemented differently in the newer Pages applications.
The current version of Pages (v14.3) is compatible with macOS Sequoia v15.3.2 on your new M4 MBP, and can provisionally open Pages '05 through Pages '09 documents. Provisionally means if there is content in those older documents that is dependent on the older applications, and is not implemented in the current Pages, then the document will either refuse to be opened, or Pages may perform automatic font replacement or other changes.
I would only open copies of your original Pages '05 - '09 documents in the current Pages as any change that you attempt to make will cause Pages to prompt you for permission to convert the document into the new Pages document format. Once that is done and autosaved, you no longer have the older Pages document that you opened. Although the current Pages has an Export To > Pages '09 feature, it is not an undo from the preceding sentence and is the only way to share current Pages content with older iWork Pages applications.
Before you install Pages from the Mac App Store and run it, I recommend that you copy the following to iCloud Drive, or a USB stick:
/Users/yourname/Library/Application\ Support/iWork/Pages/Templates
/Users/yourname/Library/Preferences/com.apple.iWork.Pages.plist
This is your old custom user Pages templates. Add the iWork/Pages/Templates content to the same location on your macOS Sequoia location. The first time you run Pages v14.3, it will convert these older custom templates to a My Templates location for the new Pages. Here again, provisional comes into play based on those custom template dependencies that may not be supported in the current Pages application. The second time you launch Pages, you will be greeted by a Template Chooser and My Templates should contain what could be converted.
The old iWork '09 Pages manual is no longer applicable. You want:
Pages User Guide for Mac - Apple Support
