Can I use .NET 4 library or Mono 2.8 in Unity ?

I have .Net 4 installed, can I used it DLL files in unity 3.1 ? or Can I use the latest Mono 2.8.1 in Unity ?

I don't quite understand which one that unity actually use, .Net or Mono ? any explanation will help. Thanks

Per the "What's new" page, Unity 3.x uses Mono 2.6.3, which is compatible with C# 3.5. From the notes, "Upgrade Mono and C# compiler to Mono 2.6.3. This brings C# 3.5, variable type inference, lambda expressions, LINQ and more. [...] Mono class libraries derived from Silverlight profile for the web player."

From the Unity website: “To create a DLL, you will first need a suitable compiler. Not all compilers that produce .NET code are guaranteed to work with Unity, so it may be wise to test the compiler with some available code before doing significant work with it.”