Apologies for the inconvenience.
The Studio One 4 Demo appeared because you don't have Studio One 4 in your account already. You can't activate Studio One 4 to full version with your Studio One 3 key, it won't work.
Possible causes why you're not seeing your upgrade in your account:
Account transaction did not go through. You can check your orders my logging into your my.presonus.com account and select the option on the tool bar for MyOrders to see if it indeed did go through. This acccount that you posted this question has your assets, if the account you purchased it through is a different one, you may have multiple registered accounts and not know it.
Another possibility is that you have more than one account, if you used a social media login (Google, FaceBook, Linked In) or a My.PreSonus.com account when you logged into the webstore, if the email address behind any one of those is not the same, you will get a new account that is not associated with your other accounts.
This means you can end up with as many as 4 different my.presonus.com accounts that are not associated with one another (if all the emails behind those accounts are not the same). Making a purchase in a separate account will not automatically be linked to your account that does have the rest of your assets. The lone account asset needs to be moved to the one account that does have all your assets.
This will require the assistance of a support agent to help sort out, this is not something we can do in a public forum, answers does not have Private messaging system and we prefer not to discuss account details in the open.
Please open a support request at https://my.presonus.com
so we can have someone look into it.