if your hosts are licensed for vMotion and meet the requirements for vMotion ... you can vMotion between 4.0 and 5.5 hosts.
Which DVD you mean if you are upgrading using .iso via i.e. ILO interface remove any media from the host physical drive...
In addition from vCenter server installation media Run the vCenter Host Agent Pre-Upgrade Checker and scan your 4.0 host for any discrepancies.
Missing your answer to VMware KB: Upgrading VMware ESXi 4.x to ESXi 5.x on a local disk fails with error: Invalid argument when creating vf…
Thanks