Section: Importing ESXi Images
With Update Manager 5.1 you can upgrade or migrate hosts that are running ESX/ESXi 4.x or ESXi 5.0 to
ESXi 5.1. Host upgrades to ESX/ESXi 4.x or ESXi 5.0 are not supported
So you cannot import your 4.x images
Regards
Girish