Personally, I've found it impossible to predict when the next bigger (or smaller), faster, better configured, etc. piece of technology will be released. Certainly, in your case, you have good reason to wonder: with Yonah (Intel's new Pentium M processor, with dual cores and better power management) being released this month (Happy New Year, everyone!), at some point new versions of many systems will be released. I'm sure the M4 will be one of them.
I'm facing the same decision, except I'm considering a Gateway M280e, principally because I have an opportunity to buy a very nice configuration for a very good price. If I didn't have this opportunity _now_, with possibly less than a month to take advantage of it, I'd likely hold off to see what's going to happen with Yonah. From what I've seen of it, its performance in many tasks will be significantly higher than with current Pentium M's, particularly video encoding--something that I'm dabbling in and that I currently don't do on my Tablet because it's just not fast enough.
Prices on Yonah-based systems will likely be somewhat higher, although I've heard that perhaps the price delta won't be that terribly large. At the same time, currently systems will likely drop in price as vendors clear out inventory. So, that gives one reason to pause in both directions--buy the newest and best later and face higher prices, or lose out on even better pricing on current systems after Yonah is released.
I'm not one for prognostication, and so I'll not even guess when new systems will be released. But, I _will_ assume that Toshiba and other vendors will wait on Yonah to refresh their lines; no reason to do so now with such a dramatic change just around the corner. So, if you want to get a sense of when new systems will be released, I'd do some research on Yonah and see what others are saying about its ETA in shipping systems. That'll probably give you as good an idea as any.
It's a tough call. Good luck!