Buy Adobe Photoshop Elements. It cost less than $100 and has 90% of the features of CS4. If you find later that you really do want CS4 you can apply the price of Elements to the upgrade. Elelements has all the tools, levels, curves, adjustment layers and so on. The user interface is the same between CS4 and Elements so the hours and weeks of study transferesover pretty well
One more thing, the best way to buy Elements is to buy a Wacom "Baboo Fun" graphic tablet. The tablet costs about the same as Elements and has a copy of Elements bundled inside. (Do check that the bundled copy of Elelemnts is the current version. Lots of old stock still around.) And the Wacom upgraded to CS4 is even a better deal than going straight to Adobe.