Either backup / restore or replication.  However, it's not something to worry about at this point.  There have been thousands and thousands of upgrades to date w/o having to revert back to 4.x. 
Hi , We are planning to upgrade firmware from 4.3.7 to 5.0.8, can you someone tell me what is reversal plan for firmware upgarde. If anything goes how we will revert back to old stage.  Thanks & Regards, Senthil Kumar D.
Downgrade Firmware in EQ PS6000