so to be clear both these Unity servers are in the same AD directory? The same users in AD are visible/importable on both systems?
If that's the case you can use GSM to move top level user data and messages from one to the other - it doesn't get everything (no notification devices, external dependencies are not preserved and such) but greetings, voice names, passwords etc... are moved.
If you were at 4.0(5) you could follow up with COBRAS to get the remaining data but that's not an option for 4.0(4).
GSM info can be found on its home page here:
http://www.lindborglabsbeta.com/Applications/Unity/GSM/GSM.html