This estimates what percentage of the trunk's configured bandwidth is currently being consumed by QoS4 traffic, the lowest-priority, most best-effort class among the four tracked here. An admin sees this class as the one that should absorb congestion first, so a persistently high QoS4 utilization alongside plenty of headroom in QoS1-3 is generally healthy, whereas a QoS4 utilization near zero for long periods might mean that traffic class isn't actually being used as intended. For example, if QoS4 utilization spikes to 100% while QoS1-3 remain low, the admin can be confident any resulting discards are hitting only best-effort traffic as designed.