Adobe Pagemaker has been discontinued years ago, but many users that were actively using it still have plenty of documents or publications saved as a .pm4 files (probably version 4).
These files cannot be opened in anything else, so the only viable way is to convert / export them to something else. Obvious conversion here is export to PDF (pm4 to pdf conversion), which can be done either through export function in PageMaker or better through a virtual PDF printer installed so you have the latest version of PDF format.Open PM4 file Open PDF file
One of the first desktop publishing programs on market
Mac version of once popular publishing program