When you uninstall the old PostgreSQL service the data folder is not automatically deleted.
We recommend that you do not delete this folder until you have fully restored your databases in the new PostgreSQL service and have confirmed that everything is working correctly.
If you experience a problem during the upgrade process and are unable to access your databases in the new PostgreSQL service you can download and install the old PostgreSQL version that you were originally using from this page, and that should allow PT4 to access your old databases again. Please contact support if you need help with this.
Once you have confirmed that your databases are working correctly in the new PostgreSQL service you may wish to delete the data folder from the old PostgreSQL service to free up the disk space.
On Windows the folder will be something like C:\Program Files\PostgreSQL\9.3\data
On macOS the folder will be something like /Library/PostgreSQL/9.3/data
"9.3" is the version number of the old PostgreSQL service, and may be different depending on the version number of your old PostgreSQL service.
Take care to only delete this folder from your old PostgreSQL service.
If there are any other files or folders alongside the data folder then that PostgreSQL version is still installed and you should not delete anything.