Adobe no longer sells CS4, outside of the exceptional cases of After Effects CS4 & Premiere Pro CS4.
For other CS4 products, if you really need them, your best bet is to check Adobe's authorized resellers.
A quick Amazon search turned up some available CS4 inventory. Just be sure what you're buying and installing is genuine and authentic, as there are a lot of fake copies out there now with hidden malware.
Hope this helps!