I haven't got time to install Movable Type 4 yet but thanks to Six Apart I've been able to access a hosted version and play a little bit with it.
My first impression is excellent, I must say that I'm impressed by the progress, no the leap, made by the MT team. MT 4 isn't a cosmetic upgrade to MT3, as we've (unfortunately) been accustomed to by 6A in the past couple of years. The impressive list of new features, the fact that the MT team has stopped diverting (therefore wasting) their efforts into two different code bases for MT and MT Enterprise, and the announcement that MT is going open source, make me think that 1) the development strategy is simpler, clearer, makes more sense therefore is more perennial than the previous one and 2) MT is no longer the ugly black duck in 6A's portfolio of products and services.
Kudos to the MT development team, and to Six Apart for reviving the potential of their “Publishing Platform”. I'm really looking forward to the production version, as well as creating some interesting blogs with it in the near future.
 The beta still bites a lot, and the interface is rough and unfinished. I wouldn't advise you to install it to replace you existing blog, unless you have time on your hands and are happy with truly beta quality (not as in web 2.0!) software.