Installers can be downloaded for free and the software will run in trial mode for 30 days then stop. The serial number is the only thing of value, as it converts the trial to a perpetually licensed version.
A serial number is what you purchase when you buy the software and Adobe isn't going to just give you a new one. If you've ever licensed then registered your CS4 with an AdobeID at www.adobe.com then log in there, again, click Manage Account, and check your list of Products. Your existing serial number(s) should be listed that you can use to relicense whatever you have installed, now.
Adobe only sells CS6 (which has stopped begin updated) and CC which is subscription and continues to be updated.