What should a 4-month-old infant be able to do when presented with affection?

At 4 months of age, infants typically exhibit social responsiveness, which includes smiling spontaneously, especially in response to affectionate interactions. This behavior reflects their developing social engagement and emotional connections with caregivers and familiar faces. At this age, they are beginning to recognize and respond to social cues, and smiling is one of the first significant social interactions they demonstrate.

In contrast, milestones such as rolling over generally commence closer to 4-6 months, while activities like stacking blocks are beyond their developmental capabilities at this stage, typically emerging later in infancy. Additionally, fear of strangers, or stranger anxiety, generally develops around 6-9 months of age, when infants start to differentiate between familiar and unfamiliar faces. Therefore, the ability to smile spontaneously in response to affection is the most appropriate developmental milestone for a 4-month-old infant.