You should have received upgrade instructions with the CD. The one CD can be used to upgrade all your installations. If you follow the instructions you will not need a new serial number to upgrade if you are installing V5 over V4. If you were to de-install V4 first or install V5 on a new PC then you would need a new V5 serial number.
Note that V4 and V5 cannot co-exist on the same PC so when you install V5, V4 will be automatically de-installed. You can open and convert all your V4 documents by going to file>Open and changing the "Files of Type" to PP4
When you have V5 installed, the Help Addendum gives you all the info on new features.
Good Luck