Believe egress priority queue doesn't have a way to guarantee bandwidth since it takes absolute priority. Are you looking to do something like CBWFQ LLQ's implicit policer?
I know you can police traffic on 3560, but recall you might be limited to policing inbound, either on a port, or with later images, VLANs. You might be able to cap your priority traffic before it gets to the egress port.
Another option, if absolute priority isn't needed, is to define the shared bandwidth allocations ratios across the 4 egress ports. Knowing the port speed, select a ratio to provide the bandwidth you need.
