Movable Type 4.0 has a totally rehauled interface. Most stuff is better, some stuff is worse.
One of the more frustrating changes to the UI, in my opinion, is that it's hard to figure out how to disable comments on a single post. For example, you might post something personal, tire of debate or get Slashdotted.
So how do you disable comments for that one post, without shutting them off site-wide?
First, log in to the MT admin interface and go to Preferences - Blog Settings:
Choose "Entries" from the list at left
and then go down to the bottom.
You will see a list of Default Editor Fields. Make sure "Feedback" is checked, and then save:
Next, navigate to the entry for which you want to disable comments. You'll notice, on the right-hand side, a "Display Options" link. Click this, and you'll see a list of Fields:
Again, make sure "feedback" is checked. You do not need to click "OK".
Now, if you scroll down to the bottom of that entry page, you will see a section for Feedback:
Un-check "Accept comments", and save the entry.
Now people will have to resort to email, or their own blogs, to yell at you. :)