vCenter Single Sign On can only manage solution users for vCenter Server 5.1. You would be able to add a vCenter Server 5.0 to the vCenter Server Web Client 5.1 but it would not be managed by Single Sign On.
You cannot add a vCenter Server 4.x to either the Web Client or the Single Sign On component of vSphere 5.1.
A possible workaround would be to disconnect the hosts from the old vCenter 4.1 and connect them into the new vCenter as vCenter Server 5.1 can manage hosts starting from 4.0. All users would connect to the new vCenter and could see the old and the new hosts.