It's like the difference between a Yugo and a Maserati.
However, CS6 was the last version capable of opening Pagemaker files, and I'm not sure it will open PM4. You also need to get your links in order before conversion, and don't expect perfection. Results of conversion can be anything tom near perfect to completely useless.
To add to what Peter said, it may require opening the PM files in the latest version of PM (which I think was 7), changing any embedded images to linked images (hoping that you still have all of the images) and doing a Save-As to clear out any residual undo data that might be in the file. Then, you would have to open those files in a version of InDesign no later than CS6 and fix anything that didn't translate well. If you don't have a machine that can run an OS that PM is compatible with, your chances are lessened.
Something to consider is how much time, effort and money (if you need to find vintage computers and software) are you willing to spend, as opposed to starting from scratch with a new ID document.
CS6 and earlier can open PM6 and later.
Download the demo and try it
" it may require opening the PM files in the latest version of PM (which I think was 7), changing any embedded images to linked images (hoping that you still have all of the images)"
FYI--you can open a Pagemaker file with embedded images and they will show (in Indesign) as embedded images--which can then be 'unembedded' and saved to a folder