Photoshop CS4 has a great new feature called the Pixel Grid. The pixel grid appears when you zoom past 500% and can help with editing at the pixel level.
One of the first changes you’ll probably notice when upgrading to Adobe Creative Suite 4 (CS4), is the use of tabs to display your windows. Previously, your windows “floated” within the CS4 interface. Now, they dock neatly up against the Options panel.