Mumakil Mahud gives characters with Mumakil in their names the carry ability. If he is within 8 squares of a stack of a Mumakil horde (M=4, or a stack of four), is that stack allowed to carry four legal characters during its movement turn?

The horde is still only one character, although one made up of four tokens, and it will only be able to carry a single other character.
At least that is my understanding.
(Also, please don't post questions in the Unit Rulings forums as very few people look there...)

In addition to what was stated, I want to point out that getting a game effect from multiple sources doesn't mean you get to stack those.

Both and give a character access to the Carry ability. gives the character possession of Carry. gives the character possession of Giant Reach which allows the use of Carry.
A character with both and still only gets to Carry one character (plus a Tiny, of course, but that's due to Tiny Size's wording).

Mama kill who? Lady be VIOLENT.