You will do an export of your 2.4(6) system and then you will need to install a 4.0(4) or a 4.0(3) version of Unity - do the restore, make sure everything is working properly and then upgrade to 4.0(5). That's the only path available to you if you don't want to rebuild your system from scratch.
As a side note, we stopped supporting 2.x quite a while ago so TAC is not going to take calls on this - but the above process does work and has been used on many,many sites. Two things that are critical and are very offten the cause of problems.
1. Your 4.0(3) or 4.0(4) install of Unity must be running properly prior to the import. This means you can create new users, import users from the directory, call in, leave a message and their lamp lights. If all those things are working, you can pull the trigger on the restore. Many folks somehow assume the restore will "smooth over any wrinkles" in their local install for them - the opposite is true. If the system is not working prior to restore, it will be a train wreck after.
2. Make sure you run dbWalker on your 2.x system prior to doing a backup and fix as many of the big issues as you can (usually missing links from deleted objects in the directory). Those errors will simply be carted over to the new box with the restore so you want to keep those to a minumum.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.