What exactly does "will not pursue" mean in Munchkin?
The monster that will not follow level 4 or below will not pursue you, only your helper will suffer bad stuff if they fail to escape. The FAQ at Steve Jackson Games covers this.

Q. I'm at Level 2 and I encounter a Level 9 monster. Another player (Level 5) joins the combat. A third player wanders in a new monster that will not pursue Level 3 or below. We are now losing and have to run away. Does the wandered-in monster go after both of us?
A. In combat, monsters fight side by side. But when it comes to pursue the fleeing munchkins, they act individually and munchkins must attempt to escape from each as normal. So the wandered-in monster will not pursue you, but will pursue your helper.