Thank you for your interest.
We are going to have a beta in about a month, then a second beta in the coming weeks. The official release is scheduled before right before Q2 though.
We will provide an upgrade tool, which will migrate all your data to the new system. This means that you can start with your project now, and migrate to 4.0 when it is available. Some of the 4.0 features are described here - http://www.sitefinity.com/asp-net-cms-features/roadmap.aspx
the Telerik team
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