The goal of SSDT is to be fully in sync with SQL Server releases. We are in the final stages of propagating SSDT CTP4 for SQL Server 2012 RC0 to the download centers, but at this point the release is not yet available.
As it stands now, all of the SSDT launch points within SQL Server 2012 RC0 point to the older SQL Server Developer Tools CTP3 Code-named “Juneau” binaries. These bits are not compatible with SQL Server 2012 RC0.
We are working diligently to correct this issue in the next 24-48 hours. After this propagation is complete and the release is live, the links to download SSDT from both the Planning tab of the SQL Server 2012 RC Installation Center and the SQL Server Database Project stub in Visual Studio will point to the newly released SSDT CTP4 bits.
We apologize for the delay and inconvenience and we will post to this blog with any release update.