Final answer:
A 4-year-old with Tourette's Syndrome is most likely to exhibit frequent eye blinking, a common tic associated with the disorder.
Step-by-step explanation:
The behavior most indicative of a 4-year-old child diagnosed with Tourette's Syndrome is frequent eye blinking.
Tourette's Syndrome is characterized by involuntary vocal sounds and movements known as tics. Frequent eye blinking is a common motor tic associated with this neurological disorder.
Other behaviors that may indicate conditions like Autism Spectrum Disorder (ASD) include impaired social skills and repetitive motor behaviors, but these are not the most indicative behaviors of Tourette's Syndrome.