You can migrate your system data (including call handlers and subscriber details etc...) using DiRT. However you'll have to migrate the 4.0(1) backup over to a 4.0(1) install and then upgrade to 4.2(1) - you cannot migrate and upgrade in one shot.
You can do this using the Disaster Recovery Tools (DiRT) - you can read the help file and watch the training videos for this tool on its home page here:
http://www.ciscounitytools.com/App_DisasterRecoveryTools.htm