Final answer:
The social worker should first conduct a comprehensive biopsychosocial assessment to gather information on the child's experiences, which could be a part of normal development or signal deeper issues, before considering further steps like play therapy.
Step-by-step explanation:
When a social worker is confronted with a four-year old child experiencing fantasies of being chased by a giant and a tiger, and having nightmares causing terror, the first step should be to conduct a comprehensive biopsychosocial assessment. This assessment will gather information on biological, psychological, and social factors that may be affecting the child's experiences. It's essential to understand the context of these fantasies and nightmares as they could reflect normal development or be indicative of deeper issues. This assessment serves as a baseline for further steps, such as play therapy, which could be appropriate depending on the findings. Additionally, providing reassurance to the parent about normal child development stages could be beneficial. However, unwarranted reassurances without a full understanding of the situation may not be appropriate.