You should use the same PT4 registration code that was initially provided to you, that code is located directly on the PokerTracker.com website. Select My Account -> Registration Codes. This same code should be used on (up to) two separate computers.
If your PokerTracker 4 registration code is not listed on the PokerTracker website under My Account -> Registration Codes then your payment email address does not match your registered website email address and you were not logged into your account when you purchased.
To solve this you can send us a support ticket by selecting Support > Contact Us while using the email address that you used to purchase PokerTracker 4 along with your full real name and we will gladly link it to your account so that you can view your code on the site.