Sorry for the delay. We have been away and when I came back my computer has had to have a major rebuild, so Iwas without it for a week :-(.
Now, for your problem. You can name the file whatever you choose. If it is say, the "Wally" family file, just name it Wally and save it with the ".paf" extension in your PAF 5 Data folder, so it becomes "Wally.paf". Having the BAK label in there is a bit confusing.
Yes, you can open your PAF 4 file while you are in PAF 5, and convert it. You will still have your oroginal PAF 4 data available after you have saved the converted data in PAF 5.