A 4 year old playing will likely be utilizing his Frontal Lobe more often when he's playing as that area is responsible for motor skills and language. When the child is actively 'playing', this area is most engaged. When children start playing pretend or with an imaginary friend, the Parietal lobe (Sensory Input, Sensory Discrimination) becomes a bit more active, and when they're cranky at the end of play time and throw a fit, that's the Temporal Lobe (Expressed Behavior, Reception of Speech) showing up.