TJ,
You can add as many LUN's to each or both host as you want to, up to the maximum for each version. The way to set this up will depend on the type of storage you're using (NFS, iSCSI, FC).
What you'll want to do is make sure they are both attached to the same vCenter and the LUN the VM is running on is available on both hosts. If you want to create a new LUN and present it to both hosts you can but you'll need to SvMotion the VM on the 4.x host to the new LUN before you try to vMotion it to the 5.x host.
-Eric