Need help with old-school install of CS4! CS2 Serial Code not being recognized

you are the unwitting recipient of illegal software. it's not your fault, but you'll be on the losing end of your purchase.

if you registered with adobe, i would expect you to hear from adobe. unfortunately, for you that may be after the return window.

if you want to get a jump. on that, contact adobe support and ask them to validate your serial number.  (unfortunately, if they do validate, that won't protect you)

there are 2 ways to contact adobe; chat and twitter:

chat:
use a browser that allows popups and cookies
and click here, https://helpx.adobe.com/contact.html?rghtup=autoOpen
in the chat field (lower right), type AGENT
be patient, it can take quite a while to reach a human.

twitter:
tweet @AdobeCare

p.s. if you're contacted by anyone (via email or private message), it's much more likely to be a scammer than an adobe representative. ie, double check for an employee badge if contacted in the forums and look for an adobe.com domain in the email address if you click reply to an email. then check again and be very suspicious. any other method of contacting (or offering to contact you) is almost certainly a scam, https://community.adobe.com/t5/using-the-community-discussions/staying-safe-in-the-adobe-support-community/td-p/12919684/redirect_from_archived_page/true