A 4.0.1 backup can be restored on the new server - even if it is running 4.1 or 4.2. I don't believe the backup will carry over anything specific to the old server's IP address. I'd recommend that you keep the same hostname (DNS-wise) - especially for things like the server's SSL certificate - although that too won't be part of the application backup so you can give the new server a new name if the customer is strongly set on that.
Note that you may change to soft appliance platform at this point. If the customer has a VMWare environment available, I'd strongly suggest considering this option. the soft appliance is a breeze to install and setup.