Darren,
Some key point to be remembered for Cisco 4500 VSS:
1) Configuration/Capability Supported supervisors on Catalyst 4500-E: VSS support Supervisor Engine 7-E or 7-LE (identical pairs).For more information about hardware to support please refer 4500 VSS Hardware requirement
2) Software requirement: Cisco IOS XE 3.4.0SG and ROMMON IOS Version 15.0(1r) SG7 later released support VSS.(Also refer How to Upgrade Cisco 4500 SUP7-E & Sup7L-E ROMMON To support VSS).
3) Minimum license : IP Base or higher (7-E) or special license (7-LE and Catalyst 4500-X).For information about Software Activation Licensing please refer Cisco Catalyst 4500E Supervisor 7-E and 7L-E and Cisco Catalyst 4500-X Series Software Activation Licensing Deployment Guide.
4) Single-sup cross-chassis VSS support: Yes.
5) Quad-sup VSS configuration with in-chassis redundant sups: In-chassis redundant sups in rommon mode with active uplinks.
6) It also supports 10 Gigabit Ethernet Virtual Switch Link (VSL) and 1 Gigabit Ethernet VSL.
7) SSO and nonstop forwarding (NSF) must be configured on each switch. If a VSS does not meet the requirements for SSO redundancy; it will be incapable of establishing a relationship with the peer switch. Catalyst 4500/4500-X series switches' VSS does not support route processor redundancy (RPR) mode
https://supportforums.cisco.com/docs/DOC-29472
HTH
Regards
Inayath
***** Please rate if this info is helpfull*****************