Sounds like you don't need the plane at all. If I understand correctly, you want to create a transformation matrix which transforms a specific oriented rectangle to the rectangle in your picture. This transformation matrix you could then also use to transform other rectangles respectively.
You can create a transformation matrix easily if you know how base vectors should be transformed. Let's use 3x3 3D matrices here instead of 4x4 for simplification. You have three base vectors i=(0,0,1); j=(0,1,0); and k=(1,0,0);. Let's assume P1 is always in the origin as it is in the picture. From the picture it looks like you want to transform j to P2 and i to P4 (the last corner, missing from the picture). You still need to figure out where to transform k, and the answer is probably the cross product of P2 and P4. Let's call it P5. Now you can just make a 3x3 transformation matrix from these transformed vectors: [P2 P4 P5]. Put them either as row or column vectors depending how you handle the matrices.
If this is what you are looking for, taking translation into account and extending these to 4x4 matrices should not be hard.