But how do we know we're not dumping slush into 2&3?
You don't but TBH even if you have it's what silverado said - Tank 1 temp reports the lowest temp fuel onboard - and TBF it's a system that has worked OK on the 747 for a few decades so if there was a tank "slush" problem in the reserves I suspect we'd have known about it now.
I flew the 744 for best part of a couple of decades, obviously (?) iaw the Fuel temp limitations ( and yes, I know they also did that on the Hatton Cross 777) and I never saw any engine probs due tank fuel temp, despite doing a lot of ULH trans Siberain/ Trans Northern Canada in winter Ops. Don't forget the main problem on the 777 at Hatton Cross was the design of the Heat Exchanger in the RR engine, not fuel tank temperature or fuel waxing in the tanks.
Does the 747 have 'stirrers' in the tanks?
Not that I've ever been made aware of.
How long does it take to cool 4000kgs of fuel from 15C to -40C?
It's been a while but I think we used to quoted 3 degrees an hour towards the TAT, though if you got into really cold Siberian Temperatures it could by more like 5-6 degrees an hour or even more.........
Just found this which might be of interest:
http://www.iasa.com.au/folders/Safet.../cold_fuel.htm