Best thing is if you can see colleagues who have each. The differences (that you'll notice) are primarily in size. The NC4400 has a smaller screen (12inch) and weights slightly less, while the NC6400 has a 14 or 15 screen. I think the 4400 is technically a "subnotebook" and the 6400 is a notebook.
Personally, I don't find the extra heft of the NC6400 noticeable, especially once you take into account the weight of the power adaptor (which is the same for both models). It also has the more usable screen.
The NC4400 is easier to sling around from meeting room to meeting room. Obviously you still need to recharge but you can run a presentation, check your email during the breaks, etc. As a main computer, I'd only use it if you can also get an external monitor, keyboard and mouse for your desk. I find the screen is too small to use for extended periods or to get large amounts of work done (for example, if you need to work on spreadsheets or construct the presentation deck before presenting it).
Obviously, no laptop screen's fantastic, but the extra few inches you get with the NC6400 I find does make a difference and is more versatile. It's impossible to get external screens when you're on client site, even if you're there for a few months, so for me the 4400 would not be a practical tool whereas the NC6400 can be used for long periods as a standalone device (assuming you're okay with using laptop keyboards).
They're both decent laptops though not as solid as Thinkpads.