[Darren69]

On my fuel sender wiring there are 4 wires but 2 are for the pump and the other 2 are presumably for the fuel sender.

The 2 black wires are both earth and appear to be shown joined together, so maybe they are. Goes without saying they are the negative.

The B/W (Blue/White) which comes from the Dashboard (maybe the lamp?)

Then there is a Bn/W which I cannot find on the key but assume it to be Brown/White which comes from the injection relay (so the switched live) so you should have 12V there when the ign is on I'm guessing.

The key actually lists Br as brown and there is no mention of Bn so your guess is as good as mine and like I say that is from the '01 S4 schematic in the workshop manual.