Much QoS does "kick in" when there's congestion, but not all. The QoS that treats congestion, generally, is queue management. (I.e. congestion is indicated by packets/frames queued.)
How you accomplish your policy of managing VoIP, ordinary business traffic and background depends on both the nature of the circuit and the QoS features of the platform.
Many LAN oriented switches tend to be feature poor for QoS. For the 4500 SupIV, there are 4 transmit queues per port, but feature restricted to some ports.
I.e.
"For systems using Supervisor Engine II-Plus, Supervisor Engine II-Plus TS, Supervisor Engine III, and Supervisor Engine IV, bandwidth can be configured on these ports only:
•Uplink ports on supervisor engines
•Ports on the WS-X4306-GB GBIC module
•Ports on the WS-X4506-GB-T CSFP module
•The 2 1000BASE-X ports on the WS-X4232-GB-RJ module
•The first 2 ports on the WS-X4418-GB module
•The two 1000BASE-X ports on the WS-X4412-2GB-TX module "
Assuming you are using one of the supported port types, and assuming the interface port bandwidth is the WAN bandwidth (often not the case), you might place VoIP traffic into Q3 configured for strict priority, replication traffic in a different dedicated queue with minimum bandwidth specified (e.g. Q1) and normal business traffic (e.g. Q2) in another dedicated queue with maximum bandwidth specified. (You'll have a queue left over that you can use later for other critical traffic.)
If your WAN path has less bandwidth than your port interface, and since the SupIV doesn't, I beleive, support queue bandwidth allocations within a shaped rate, I would recommend you drop another device in-line (even a 8 port 2960 would help).
More information can be found staring in the configuration guide for your IOS in the chapter "Configuring Quality of Service".
PS:
More QoS recommendations for 4500s:
http://www.cisco.com/en/US/docs/solutions/Enterprise/Video/tpqoscampus.html#wp1062354
http://www.cisco.com/en/US/docs/solutions/Enterprise/WAN_and_MAN/QoS_SRND_40/QoSCampus_40.html#wp1099634
http://www.cisco.com/en/US/docs/solutions/Enterprise/WAN_and_MAN/QoS_SRND/QoSDesign.html#wp999958
