Full UV wrapping on a mirrored Mesh I have an object with a mirror modifier in X and Y direction. Is it possible to unwrap all the 4 quadrants as a big single UV map and then have this UV map to apply to the object as it was a single non mirrored object (Same as apply in the mirror modifier), without using the Apply button first?

I wan't to be able to edit the mesh with the Mirror modifier later on, but have the UV map seeing it as one single object.
Yes, this is possible (out of the box).  The mirror modifier has a mirror UV option, so you can mirror either the U or V (or both) texture coordinates.  So for example, you can model the left hand side of a head with the mirror modifier enabled for X and U.  Then unwrap it to the left-hand side of your texture.  The mirrored right-hand side of the mesh will have its UV co-ordinates on the right-hand side of the texture, so you can have something different there, e.g. a scar drawn on the texture's right cheek which appears on the right cheek of the mirrored model.