To have QoS active on a physical link that has more bandwidth then the end-to-end path provides, you often need a shaper.
e.g. (CBWFQ pseudo config)
service-policy aShaperParent
class class-default
shape average 4Mbps
policy aQOS_Child
service-policy aQOS_Child
class class-default
random-detect
interface fastethernet 0
service-policy aShaperParent