Is it possible to add ESX4.0 host into vCenter 5.5 along with all the virtual machines running on esx4.0 host?
Yes, vCenter Server 5.5 supports ESX/ESXi hosts with versions from 4.0 up to 5.5 (see VMware Product Interoperability Matrixes). It should work to disconnect and then remove the hosts from the old vCenter Server, and then add them to the new vCenter Server with the VMs still powered on.
What you should do, is to disable HA on the old vCenter Server prior to moving the hosts to the new vCenter Server. You may - depending on your configuration - also want to preconfigure clusters/settings on the new vCenter Server (e.g. EVC mode settings).
Btw. are you using vSphere Distributed Switch (VDS), NEXUS 1000V, IP-Pools or other features and add-ons? In this case it won't be a simply disconnect/reconnect from/to the old/new vCenter Server. Also remember that you may need to reconfigure tools like image based backup applications!?
André
