C# 4.0 support on windows only via .dll ??

On a windows only setup, is it then possible to get full C# 4.0 support through a linked .dll or is the plugin recompiled by Unity to fit Mono?

Using a compiled DLL won't help you and is likely to give you trouble. However, Unity 3 will support most .NET 4.0 features so when you use Unity 3, you should be able to use .NET 4.0 DLLs both on Windows and Mac (and likely also on other platforms like iPhone or Android; however, you need to be careful with dependencies and not all features might be fully supported on the mobile platforms that require AOT compilation).