This means you can continue to tweak the adjustment sliders even after you've finished painting. You can click the little plus and minus signs on either end of the adjustment sliders to lessen or strengthen the adjustment, turn on the option to view the mask as well as change the mask colour. By enabling Auto Mask you'll confine your adjustment to brush strokes that fall upon areas of similar colour. Masks are fully editable and you can make as many masks as you want or delete them.
Also new is the ability to apply adjustments using a Graduated Filter tool, much the like the real graduated filter you'd screw onto the end of your camera lens. You get the same set of adjustments as you do with the Adjustment Brush, though with the Graduated Filter tool you apply them by clicking and dragging (you're really doing is creating a gradient mask, which hides the adjustment from the rest of the image). You can continue to make adjustments even after you've used the tool.
In order to intensify the sky gradually, you can use the new Graduated Filter tool in Camera Raw. The green dot represents the start of the mask and the red dot represents the end.
New Extended tools
Photoshop CS4 Extended users will notice a couple of new tools called 3D Rotate and 3D Orbit in their Tools panel, as well as an entire menu devoted to the realm of 3D which includes New Mesh from Grayscale, Invert Visible Surfaces, and Create UV Overlays. If you've ever dreamt of dabbling in 3D modeling, Photoshop Extended is a great place to start. Just like you can buy stock images, video and audio clips, you can also buy 3D objects and import them into Photoshop (just do a quick Google search and you'll find oceans of 3D models to experiment with).
What's truly amazing is that if the objects are already painted and lit, you can import that info into your scene, completely intact. You've also got a fair amount of control over the object's textures and lighting too. If the object came with a separate texture file, you can edit it in Photoshop and see the changes updated on your model when you save. You can also add and adjust lighting to make the model better blend into your scene.
If you're feeling particularly brave, you can paint directly on the surface of a model in real time. For 2D folks this may sound like magic, but for experienced 3D jockeys it's old news. Though it's really nice to have this functionality in Photoshop, especially if you don't have a 3D painting program.
If you're a video editor, you may appreciate the single-letter keyboard shortcuts, smooth display of non-square pixels, and the ability to preview and export audio.
Of course there are also tons of other additions like the ability to delete layers by pressing -- wait for it -- the Delete key (yay!), enhanced Auto-Align and Auto-Blend commands for creating panoramas, complex composites, and extending your perceived depth of field through merging several images of the same object with different focus points. You'll also find support for Multi-Touch trackpads on Mac laptops, colour profiles for the colourblind, a 6-colour histogram in the Histogram panel, support for 16-bit printing, and a new online help system.
As mentioned earlier, while Photoshop CS4 is indeed 64-bit compatible on the Windows side, Mac users will have to wait till CS5. While 64-bit compatibility was on Adobe's road map, it was developing 64-bit compatibility into the existing Carbon-based program. At its World Wide Developer Conference in June 2007, Apple announced the halt of Carbon 64 development and the intention to forge ahead with Cocoa instead. That means Photoshop for the Mac needs to be rewritten from the ground up, and Adobe says they're working on it fast and furiously. Though honestly, 64-bit is only meaningful if you work on files over 4GB in size -- the vast majority of us don't.
...And taketh away
With the good comes a little bad: To make room for new features, some long-time, built-in plug-ins hit the cutting room floor, like the Extract Filter, Picture Package, Contact Sheets, and PDF Presentations. Thankfully, the older versions of those plug-ins still work (save for PDF Presentations) and you can download them from Adobe's site or snatch them from your copy of Photoshop CS3. The drag and drop PhotoMerge dialog box has also passed quietly into the night, along with the ability to create a PDF of a Layer comp within Photoshop (though you can still do it in Bridge).
All of the changes in Photoshop CS4 and Photoshop Extended CS4 add up to a friendlier, more accessible interface (and with Photoshop that's an accomplishment), with increasingly helpful tools that are much easier to find and use than before. The changes may take some getting used to, but in the end, you'll come to appreciate the more streamlined and uncluttered experience.
If you're a photographer, the enhancements in Camera Raw alone may well be worth the upgrade and for everyone else, the additions along with the improvements to existing tools makes for a much more enjoyable Photoshop experience.