EX4300 platforms only support three different packet loss-priorities (PLPs) per forwarding-class/queue at the hardware level. Nevertheless, it is still possible to use all four PLPs (low, medium-low, medium-high and high) within the CoS configuration. In that case, PLPs medium-low and medium-high are used for the same hardware-level PLP. In more detail, packets that are classified with PLP medium-low are internally mapped to PLP medium-high.
This behavior isn't documented yet. A corresponding support case has already been opened.