It is a confusing topic, and it isn't always cut and dried, but for the S4 it is pretty straightforwards - all of the ECUs are (hardware) compatible as long as they have the same flash size.
For A6/allroad it is trickier because ME7.1.1 is not hardware compatible with ME7.1
http://files.s4wiki.com/stock/part-numbers.txtgotchya
i won't be worried for the a6/allroad ecus/files as they wont apply to me. either way the info is nice to know
but the s4 info will be used
thanks a lot