Final answer:
A 4-month-old infant can hold their head at a 90-degree angle while prone. They are not yet at the developmental stage to crawl up stairs, creep on the floor for lengthy distances, or walk upright with a waddling gait.
Step-by-step explanation:
When assessing the developmental milestones of a 4-month-old infant, a nurse would expect the infant to have certain capabilities. By the end of the fourth month, an infant can typically roll from front to side, lift their head 90 degrees while lying prone, sit up with support, and hold their head steady for short periods. The ability to crawl up stairs, creep on the floor at least 30 feet, or walk upright with a waddling gait are milestones that are reached much later. Specifically, the correct answer is that a 4-month-old infant can hold their head at a 90-degree angle while prone. This reflects the infant's neck and upper body strength development typical for their age group.