even if the CE4.0 sounds like it is a .NET 4.0 only assembly, that is not true. You can use it in 3.5 or 4.0 applications. Only the EF part (what we do not use) is .NET 4.0 only.
The OpenAccess 3.5 extensions assembly contains mainly our Linq implementation, it has to be used in 3.5 and 4.0 application. In our current release the 4.0 assembly contains only our Data service v1 implementation and my best guess is that you don't need it referenced.
All the best,
the Telerik team
Q3'12 SP1 of OpenAccess ORM packs Multi-Table Entities mapping support. Check it out.