Got beta 5 of Movable Type 4 up and running. So far so good. All the issues I was having with beta 3 have been taken care of and I haven't run into any significant new ones yet.
The default templates that come with MT4 are very different from previous MT templates. I haven't had a chance to go through them all yet, but everything has been made more modular, which should make things easier to troubleshoot. It does increase the complexity and number of templates and modules to deal with though.
MT4 is a little more sluggish compared to MT3 when moving around in the back end. I suspect some of that can be alleviated if I moved to a mod_perl or mod_fcgi environment. Didn't have much luck with doing that with MT3, but support for them is supposed to be better in MT4 so maybe I'll give it a try again.
Depending on how beta 5 goes, I might go ahead and migrate the blog over to get a better idea of how things go in the real world.
Update: The MTEntryIfComments plugin seems to have broken with b5 with the following error on rebuild:
Parse error in template 'Main Index': <MTEntryIfComments> at line 129 is unrecognized.
Fortunately it can be easily worked around by using a
<mt:if name="comment_count" gt="0">
type of construct, but now I'm finding that MTEntryCommentCount always evaluates to 0 in beta 5. Didn't notice if this worked before, but I submitted a bug report so I should find out soon if it's a problem with me or with MT.