I do not know i you have reveiwed the upgrade guid - http://pubs.vmware.com/vsphere-51/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-51-upgrade-guide.pdf - it will step you through what you need to do -
You have dobne the first step by upgrading your vcenter sevrer - the process to move to the environment is simple-
1) evcuate the vms form the first ESXi 4 host and place it into maintenance mode -
2) remove that host from your vcenter environment
3) shute it down
4) bring up one of the ESXi 5 hosts
5)add it to the vcenter environment
6) apply the hhost profile
7) move vms to the new host
8) and repeat the process -