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

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.

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.