Log in to your Reallusion Member Account and navigate to the "Registration" or "My Products" page. You can click "Resend" to have the serial number displayed on your screen.
While some sites claim to offer "cracked" serial numbers or keygens, using these carries significant risks: Character Creator 4 - Is it Worth Buying? (Review)
A Character Creator 4 serial number is provided upon the purchase of a software license. Reallusion currently offers several ways to obtain and manage these keys: