From presonus.zendesk:
"Please do NOT enter a product key or check the Upgrade box. Run Studio One 4 and follow the steps below...
1. Go to menu Studio One > Studio One Activation (if it launches directly into the activation screen go to step 2)
2. Check the option ... Activate my purchased version of Studio One. Do not check the box "I have purchased an upgrade of Studio One”
3. Click the downward facing arrow on the right edge of the list field there to refresh your product keys listing with Show My Existing Product Keys. This will retrieve the registered products from your user account.
4. Select Studio One 4 Professional from the refreshed list and click Activate and it will activate the new license and restart as Studio One 4 Professional.
"
