VSS only allows you to aggregate two physical chassis at a time. Remember there are really only two main drivers to even using VSS (and many arguments against it depending on who you ask):
- Etherchannel aggregation across multiple physical devices for hardware redundancy.
- Ease of management/configuration by consolidating the two chassis into a single point of management.
In a "box" design like this, comprising four switches, you would use VSS on each pair separately. Then use the normal network protocols (Spanning Tree, routing, etc) to manage traffic to/through the other pair.
Based on your questions, I thought it might be good to clarify a few basic components of VSS.
First, While it does logically aggregate two separate physical chassis, the way it does so for the Data Plane and the Control Plane are different. In a VSS pair the Data Plane is active on both switches, however from the Control Plane/NMS perspective only one switch is active and responding. This is what gives VSS it's single point of management.
Secondly, the VSL interfaces are only used among a single pair of switches. To cable two pairs of VSS Switches together (for a design like you reference), you would simply use existing ethernet ports and configure 802.1Q trunks or routed interfaces as needed in your design. Each of the two VSS pairs would appear as independent switches to the other. No multi-chassis Etherchannel or single point of management functionality is possible between the two VSS pairs, they are for all intents and purposes two separate switches.
See this very helpful VSS Q/A document that Cisco has put together, or the Cisco Solution Overview document for VSS for more information.
Best Answer
Edit: After further research, my original answer is not exactly accurate. It looks like Cisco TAC would like you to use the same hardware, but I believe it is not a requirement.
See the 6500 VSS FAQ, which was indicated to also apply to the 4500 series.
I stick by the rule of thumb though, from discussion with Cisco and other people in the last few years, using identical hardware tends to result in a better VSS experience.
Original Answer:
The switches should be the same model.
A 4500E should only form a VSS pair with another 4500E, and likewise for the 4500X.
Best rule of thumb for making VSS function properly: Use Identical Hardware.
I've heard anecdotal evidence that VSS can be forced to work in other configurations (that is to say, it may be allowed via software), but I doubt Cisco TAC would support them.
For more in-depth detail, see this doc from the Cisco Support Communities.