Yes, that is right Cathy. PAF 4 will not open your PAF 5 files now they are "converted". However, you can still open the PAF 4 files in PAF 4.
When you asked PAF 5 to "convert" your PAF 4 files did you remember to "save" the converted files (PAF 5) to a new folder as I suggested?
Or, did you just click "SAVE"?
This would have resulted in your PAF 4 files being "overwritten" and impossible to re-open in PAF 4.
You did mention that you still have backups of your PAF 4 files on a CD. You will still be able to open those in PAF 4, if they are needed.
Both programs will run independently of each other, but entries have to be made in both to keep them up to date (I would suggest just begin using PAF 5 and leave PAF 4 there for the time being without using it).