When linking a .NET 2.0 Managed Assembly from a .NET 4.0 Application, which framework is used?

If I have a 2.0 CLR assembly (pure managed code, no mixed mode issues) that I need to link to from a 4.0 CLR Application, does the 2.0 code run on the 2.0 CLR or 4.0.

Basically, is there any risk of 4.0 breaking changes affecting the 2.0 code?