I want to develop my own AR-Library in C#. My problem is: I have the 4 corner points of my marker and want to show 3D cubes on the marker (it's a multi-marker lib), but I don't know how to get the matrices for xna.
The term you are looking for is called "3d pose estimation". Have a look at this link: http://www.aforgenet.com/articles/posit/ . It describes the POSIT algorithm and there's also a sample application including source code you can download. Conveniently for you, it's also written in C#.