This read-write value sets the portion of the port's data rate reserved for ATM QoS Class 4 traffic, generally the lowest, best-effort priority tier, so even this class gets some guaranteed minimum capacity rather than being entirely at the mercy of the higher classes' demand. An admin usually sizes this conservatively since class-4 traffic is meant to use whatever capacity remains after higher classes are served, but a nonzero reservation here still protects it from complete starvation. For example, setting a modest lportAtmDataRateQoS4 value ensures bulk file-transfer or best-effort browsing traffic assigned to class 4 always gets at least some throughput even during sustained peak demand from classes 1 through 3.