Final answer:
A four-year-old child who starts wetting the bed after a new sibling's birth is likely displaying the defense mechanism of regression, reverting to behaviors of an earlier developmental stage.
Step-by-step explanation:
The four-year-old child who begins wetting the bed after the arrival of a new sibling is likely exhibiting the defense mechanism called regression. Regression is a behavior where an individual reverts to an earlier stage of development and demonstrates behaviors that were typical of that stage. In this case, the child may be regressing to an earlier stage that requires less responsibility and receives more parental attention, possibly due to feeling threatened by the newborn who naturally demands significant parental care.