You have got 4 abs sensors, one for every wheel. they are behind the brakes, if you remove the wheel you will see the brakeline (rubber hose) and a cable wich goes to the sensor
SOURCE: Abs and stop lights are
These lights mean the ABS system has been shut off due to a fault, the brakes will fell fine but the ABS is not going to work if you need it until you fix the fault.
SOURCE: temperature sensor audi a4
Possibly located at the back of the engine behind cylinder head, usually located in a small plastic hose block.
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SOURCE: ESP ABS LIGHT ON .AS WELL MY BREAK LIGHT FLASHING.
i have seen this many times on VW passats which are on the same platform. You may need a new abs control unit. if the abs control unit cannot be scanned by a computer, check all fuses. if abs contol unit is needed, there is aftermarket places that can rebuild them, i recommend factory unit.
SOURCE: where is the 2004 audi a4 rpm sensor
For Diesel and also for Otto engines on 2004 Audi A4, this rpm sensor (impulse sender - crankshaft) is placed on the bottom left back side of the cylinder block. 
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SOURCE: A4 airbag sensor
A4s are known for airbag lights due to the bag in the side of the driver's seat. The yellow connector under the seat gets stretched and opens up partially if you move the seat too far. It's just a quick opening but it's enough to trip the light. In mine, I ziptied the connector tightly together, then turned off the light (you need the VagCom diagnostic software for this).
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