The best option for you is to upgrade the website to 4.1 and then to 4.2 SP1 as there were hundreds of changes throughout the releases which might cause some problems if you upgrade directly to 4.2 SP1. You also need to download and activate new licenses with each new major release you install. If you need more guidance through the upgrade process feel free to ask.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items