You can download CS4 software thru the following page:
If you do not have the serial number from the disc, if you registered the software your serial number should be available thru your Adobe account online. You might find download links available there as well.
You couldn't purchase a new copy since Adobe does not sell older versions of their software.
Thank you so much for your prompt response. I have now managed to download the software, which is just brilliant!
You're welcome Diane