This question already has an answer here:
Is it possible to use Roslyn compiler and new features of C# 6.0 with old versions of .NET Runtime (for example, .NET 4.0)?
For example, I want use the expression-bodied members (int S => x + y; instead of int S { get { return x + y; } }) in .NET 4.0 application.
The new C# 6.0 features don't depend upon framework support so yes, you an app compiled with the C# 6.0 compiler will run on .NET 4.0, assuming of course you specify you are targeting .NET 4.0 in your project file in the first place.
As of now Roslyn C# compiler and tooling cannot run on .NET 4.0
You can certainly cross-compile for .NET 4.0, but to run the compiler itself you need at least .NET 4.5
