Esxi 5 license keygen

There is a field called "License Version" when you decrypt license keys. This shows where the license keys are applicable. For all 4.x based licenses, the LicenseVersion = "4.0". For all 5.x based ESX, LicenseVersion = "5.0". So you cannot use them interchangeably. You need to purchase a license for your 5.0 host or go with the free hypervisor license (in this case, you won't be able to add it to vCenter. Hope this helps.