You will need to log in to your ShaderMap account to download the key. An email should have been sent to the email address you gave PayPal at checkout. Log in here: https://shadermap.com/downloads
That is odd. You can still log in with the email address you gave to PayPal. To get a new password, enter that email address here: https://shadermap.com/downloads/_reset.php
A new password will be sent to you. Once you receive it, you can log in at https://shadermap.com/downloads/
Then download the SM4 PRO key file and double click it or drag it on to the SM4 shortcut.