Is it possible to combine a 4500E with a 4500X into a VSS pair or do they have to be the same model to pair up? If it requires same model, is it due to hardware or software? For example if it is something in software maybe in the future the two different models could be paired up.
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.
Q. How many Cisco Catalyst 6500 Series Switches can be part of a VSS?
A. The VSS1440 will allow up to two physical Cisco Catalyst 6500 Series Switches to operate as a VSS. The two physical chassis does not need to be identical in the type of modules installed or even type of chassis. For example, a WS-C6503-E chassis can be combined with a WS-C6513 chassis to form a VSS.
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.
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.
Though you can have the same supervisor in each switch, the key requirement for VSS is that both chassis have the same number or slots. In this case the fixed 4500X and modular 4500E will not, and cannot be put in a pair. Hardware requirements can be found here: http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/15.1.2/XE_340/configuration/guide/vss.html#wp1053927