Can I use both my Sitefinity 3.x license key and my Sitefinity 4.x license key for the same domain?
Yes, you may use either one of the licenses for the same domain name. If you have a Sitefinity 3.x license with an active subscription, then the key for your Sitefinity 4.x Standard Edition has already been automatically activated in your client account. You should be able to see both keys in your account, one for Sitefinity 3.7 Standard Edition and one for Sitefinity 4.x Standard Edition.