WE version has a new chassis that has a lower center of gravity, it is mostly graphite parts, and heavier duty belts. Also includes most of your normal Factory Team type upgrades. Titanium turnbuckles, hinge pins, threaded shocks, and different rear arms. There might be a few other differences I am not remembering.
Chances are any old XX4 you buy that has been used until recently and not just sitting in someones workshop for the last few years will have the new belts and most of the graphite suspension parts.
For the price difference you normally see on E-bay between the WE and the older version I would go with (and did) the older version for a lower price. I did upgrade to the new chassis which had great results for me. Didn't really notice a difference with the new rear arms. The threaded shocks and Titanium parts are nice but in my opinion are not really neccessary. Though the Titinium tends to bend rather than snap so you can at least finish the race.
Hope this helps. There was an older thread on this but it looks like anything with posts older than 2/07 have been deleted.