This reports the current depth, in packets, of the priority-4 (lowest priority) transmit queue on the port, the class most likely to be discarded first under lportMildThreshold and lportSevereThreshold congestion rules. An admin checks this queue specifically when troubleshooting complaints from low-priority applications, since it is expected to grow and shrink more than the higher-priority queues as bandwidth is rationed. For example, seeing lportQP4Len pinned at its maximum while other queues are empty confirms that priority-4 traffic is effectively being throttled to make room for higher classes.