There's no "urea quality sensor" -> the quality of the urea is assessed by changing the duty cycle of the metering unit and observing the shift in the post-SCR NOx sensor.
The 4 wires are all for the level sensor: there's a common, and 3 wires for the "FULL", "HALF" and "LOW" probes.

There is a pump, actually 2: there's a transfer pump and a supply module. The first one simply moves fluid from the passive to the active, the second one pressurizes the line going to the "metering unit" which is nothing more than in injector with some cooling fins.

Great work on this!!!!!

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