No. According to the following Q&A link (http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps9336/prod_qas0900aecd806ed74b.html)
Q. What are the hardware and software requirements and restrictions for VSS?
Virtual Switching Supervisor 720-10GE (VS-S720-10GE-3C and VS-S720-10GE-3CXL)#
Note: Initial release supports a single supervisor engine per virtual switch member (or chassis). This restriction will be removed in a future software release.
The current VSS is currently as follows (http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/vss.html#wp1054558)
A VSS operates stateful switchover (SSO) between the active and standby supervisor engines. Compared to standalone mode, a VSS has the following important differences in its redundancy model:#
•The active and standby supervisor engines are hosted in separate chassis and use the VSL to exchange information.
•The active supervisor engine controls both chassis of the VSS. The active supervisor engine runs the Layer 2 and Layer 3 control protocols and manages the switching modules on both chassis.
•The active and standby chassis both perform data traffic forwarding.
If the active supervisor engine fails, the standby supervisor engine initiates a switchover and assumes the active role.
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I haven't tried it, but since it's what you're left with if one of your quad sups fails, one would expect it to work.