Re: VC 5.1 Update manager

http://pubs.vmware.com/vsphere-51/topic/com.vmware.ICbase/PDF/vsphere-update-manager-51-install-administration-guide.pdf

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