If a creature you control with power 4 or greater has menace, it can’t be blocked by only one creature and it can’t be blocked by more than one creature, so it simply can’t be blocked.

Once a creature you control with power 3 or less has become blocked by two or more creatures, changing its power won’t cause either blocking creature to stop blocking it.

Challenger Troll’s ability affects itself as long as its power remains 4 or greater.