There is no need to install PP3 on the new workstation. You will install the upgrade (PP4) directly using the new serial number. After that, you will proceed with the activation process.
If you want to use pp3 forms you have to import them with PlanetPress4.
To access the PP3 Watch processes, you should open the Watch config file (config.pw3) in PPWatch4.
To be able to access all previously compiled documents, you may want to copy the contents of
PlanetPress Suite 3/PlanetPress Watch/FORMS
PlanetPress Suite 4/PlanetPress Watch/Documents