Adobe has officially launched Creative Suite 5, so why would you be interested in upgrading to CS4? The answer is system requirements. CS5 requires an Intel (64-bit) Mac running at least Leopard (OS X 10.5.7) or later, while CS4 can run on both Intel and PowerMac.
By upgrading to CS4, you can continue to use your PowerMac G5 or iMac G5 as your design workstation. Since many Creative Suite users tend to skip one product cycle, there are bound to be many of you still using CS3. When you consider the cost of upgrading to CS5, plus the added cost of a new Intel Mac, Creative Suite 4 starts to look very attractive.
And once you tell your boss that you just saved the company a few thousand, you may want to see if you can get the OK to beef up the memory in your current Mac.
Adobe CS4 licensing upgrades are no longer available as of 26th April 2010 @ 4pm.
However, you can still get CS4 as a boxed product for the time being.
If you plan to stick with your G5-based Mac for a while, and you're using Creative Suite 2 or 3, consider upgrading to CS4. It can keep you productive while you beg your boss for the latest Intel Mac! If you don't upgrade to CS4, then you'll be stuck in time.