I had this same problem a year ago when switching laptops and no longer had my CS4 discs.
The blog post above shows you how to use your Adobe account and its cookies to safely and legally download from Adobe servers the version of your choice. It is essentially the same thing as downloading the trial version except you will be required to put in your serial number before you can use it at all.
To me, this seems the only legal way to get their old software and I would only suggest this to be used by people who already have a serial number for the version they download. There's no hacking, no torrenting, no cracking. It's merely a step-by-step process to ensure you have the right session cookie for you to access the files from a direct download link hosted by Adobe.