There are many variables such as if you are deploying unified messaging or voicemail only, and if you are deploying a failover server as well.
The following link provides installation guides for each version of Unity depending on how you plan to deploy it. For Unity 4.2(1) you will need to find the proper 4.0(5) installation guide and also reference the 4.2 addendum for differences in the installation of 4.2 vs 4.0(5). Hope this helps.
