Challange Rating (CR) is an aggregate score of several statistics in a monster's stat block. A creature's Challange Rating is the average of two measurements: offensive challenge and defensive challenge. Each of these two categories have various characteristics, measurements, and weights affecting their final calculation.
This calculation should be a challenge equal to 4 equivalent Player Characters equal of the same level, that should technically should not result in any Player Character's reaching the Dead status if both side are starting at full resources.