Well, the main benefit about Tomcat 4 is that it supports the latest Servlet/JSP specs. So you could do a lot of cool and useful things that couldn't be done before. Number 3 and 4 should be basically the same between versions 3.2 and 4.x.
As far as points 1 and 2, I would say it is a bit early to tell. I would definitely think 3.2 is a bit more stable only because it has been around for a long time while 4.x did not go final release until this past month. Performance may be a bit better in 4.x.
Support is definitely going to be better in the short-term with 3.2, until more people start using 4.x. Long-term, 4.x is definitely the way to go.
Overall, I would go with 4.x. Just know that it is a recent release so all the bugs might not be worked out. Than again there are known bugs in 3.2 and those will never be worked out. Wushutwist