PAF 5 uses a different database format than PAF 3/4 You can use automatic conversion from PAF 5 or GEDCOM process from 3/4 to 5.
Being you don't have 4 available, you could either download it from www.familysearch.org, or you could try to find someone with PAF 4 and restore it, create a Gedcom file of it, or if that person should also have a copy of PAF 5, they could convert the PAF 4 .paf to the PAF 5 format.
I have both, if you would like me to do either for you, I would be happy to. Just send it to me email and attach your PAF 4 backup. I will delete it off my computer when finished if that is one of your concerns.