This sets the peak cell rate for rate queue 4 on this port, the first of the four low-priority queues (4-7) per the description. Traffic mapped to this low-priority queue on this port is expected to tolerate more delay and loss than queues 0-3, so this ceiling is where an admin caps best-effort or data traffic to keep it from crowding out the high-priority queues sharing the same physical link. An admin might set pportPeakCellRate4 aggressively low relative to queue 0 to guarantee this port's voice traffic in queue 0 is never starved by bursty data mapped to queue 4.