This read-only value reports the currently available outgoing virtual bandwidth within the QoS Class 4 reservation on this port, the unused headroom from lportAtmDataRateQoS4's reserved capacity for that lowest-priority class. An admin checks this less urgently than the higher-class equivalents since class 4 is typically best-effort, but it still matters when trying to understand whether the port's overall reserved capacity across all four classes is nearing exhaustion. For example, comparing lportOutVAvailbwQoS4 against the other three QoS available-bandwidth counters gives the admin a full picture of which class's reservation is closest to being fully consumed on this port.