I've been trying to do the same thing, but haven't had any success... It seemed like it would be pretty straightforward:
CODE
<mt:Entries sort_by="comment_count" limit="4">
...
</mt:Entries>
Not only does it not get the 4 most-commented posts in that blog, the articles that do get displayed aren't even sorted by their comment_count. (btw - I'm using MT 4.13)
If anybody has any insight on a solution, it would be greatly appreciated!