There is no direct "GNS3-ready" download for Catalyst 4500 software because these images are copyrighted by Cisco. To legally obtain any Cisco software, you must:
Router> enable Router# vlan database Router(vlan)# vlan 10 Router(vlan)# vlan 20 Router(vlan)# exit cisco 4500 switch ios download for gns3
This is the modern standard for Cisco switching in GNS3. It is a virtualized Layer 2 image that supports most features needed for labbing. You can find appliance details on the Cisco IOSvL2 - GNS3 marketplace. There is no direct "GNS3-ready" download for Catalyst