L4 expectations: Design a working system with reasonable components. Identify the main bottleneck. Discuss one trade-off.
L5 expectations: Design a system that handles Google-scale traffic. Go deep on - components. Discuss multiple trade-offs with data to support your choices. Mention specific technologies and explain why (e.g., "Bigtable over MySQL because of write throughput at this scale").
L6+ expectations: Everything L5 does, plus: anticipate future requirements, discuss organizational trade-offs (team ownership, deployment complexity), and propose monitoring and alerting strategies.