You can search back through this forum (from around 2001) for info on conversion from PAF4 to PAF5.Here are the notes from the PAF5 Help files:
You will need to convert the .paf files from previous versions of Personal Ancestral File and from other programs to use them in Personal Ancestral File 5.0.
Personal Ancestral File 2.0 to 2.31. If you have information in versions 2.0 to 2.31, you can either (1) convert the file into Personal Ancestral File 3.0 or 4.0. Convert the resulting file to version 5.0 or (2) create a GEDCOM file, and import it into version 5.0.
Personal Ancestral File 3.0 and 4.0. You must convert the .paf file before you can use it in version 5.0. During the file conversion process, you can save the converted file in a different folder and give it a different name. You can still use the original file in Personal Ancestral File 3.0 and 4.0. -------------
Since PAF is a free program, it doesn't cost anything to stay up with the current version.Having the latest version always makes it easier to get help with any problems you might have.I've had the lastest version for quite a while, and don't remember what the exact changes were - I'm sure it's posted somewhere on the web - but I've used PAF for MANY years and am still happy with it.