Zoom has made a number of significant changes to the Zoom H4 design for the H4N. For starters, the new model has a rubberized grip which makes it feel like a much more solid device. The older model feels like cheap plastic by comparison.
The H4N also has a much larger, easier to read display and the button layout makes the menus much easier to navigate. The built in stereo condensor microphones can also be rotated to adjust the pickup pattern. And you can now record four separate tracks simultaneously by using the two built in mics and the two XLR inputs.
Probably the most important change is that Zoom says it is using better preamps for the XLR inputs. I always thought the Zoom H4 sounded excellent when you use the built in mics, but the moment you plug a microphone in, you wind up picking up a hissing noise. I wasn't able to test the preamps on the noisy show room floor, but hopefully I'll be able to get my hands on a review unit soon so I can see how the H4N stacks up against the Zoom H4 and the Sony PCM D-50. I wouldn't really expect it to sound as good as the Sony recorder which sells for over $100 more. But if it comes close, this recorder could be a steal at $349.
You can check out some photos I shot of the Zoom H4N as well as some images comparing it to the original H4 below:
Update: You can also find the official spec sheet and some higher resolution images here.
And here's a short video I shot with a Samson representative about the Zoom H4N: