I would like to activate CS4 after I formmated my computer, but it fails since I've already
done that several times. How can I activate it?
it's a software I purchased several years ago and I still have the serial number
Contact Adobe Support thru chat and ask them to reset your activations. If they are not able to they should be able to explain why it is not possible for you to reactivate.
Chat support - For the link below click the Still Need Help? option in the blue area at the bottom and choose the chat option...
Make sure you are logged in to the Adobe site, have cookies enabled, clear your cookie cache. If it fails to connect try using a different browser.
Serial number and activation chat support (non-CC)
http://helpx.adobe.com/x-productkb/global/service1.html ( http://adobe.ly/1aYjbSC )