As a rule,part of Adobe products new features come from top 100 features requested by users and other part is completely new features to let Adobe marketing better sell its product (don’t forget that Adobe is a business company first of all). In addition, the Photoshop developers are working hard to make the 64-bit version of Photoshop CS4 (codename Stonehenge) for Windows with significant performance kick for some tasks. While there is a long way before Adobe Photoshop CS4 final release I tried to pick all available currently info about new features.

Enhanced masking feature
Layer Mask Density option allows you to fade away the effect of a layer mask gradually revealing the parts that are hidden.

Real-time filters with no color depth restrictions
New Adobe technology called Hydra allows filters to use graphics card power in image processing instead of the CPU. That means all operations will be much faster. Hydra filters support all color depths as well.

Liquid resize feature
This new technology called Seam Carving allows you to shrink or enlarge a photo disproportionally without stretching it.

Enhanced Quick Selection tool
The Refine Edge dialog now has a new feature to deals much better with complex surfaces such as fur ensuring a more defined line between subject and background.

Improved Dodge and Burn tools
Improved algorithm in these tools lets results look less destructive and more natural.

Polished interface
Photoshop interface brightness will be adjustable like in Adobe Bridge with buttons and dialog boxes look will be improved.

Extended and enhanced color profile management.
Multichannel, Device-Link and Abstract color profiles to be added.

Other Photoshop CS4 new features and improvements:
Integration of xRez technology for creating amazing panoramas.
integration of KnowHow and Kuler into Photoshop.
Rotatable canvas like in Corel Painter.
Better HDR support.
Faster load time.

Source : [link]