Theoretically you should be able to load 20,000lbs of fuel, so far, in well over two years I've never been able to do this, despite following all the advice of pumping the aux tanks slowly and getting the plane 1 degree nose down etc. The high level shut-offs cut out well before 20,000lbs, at VERY best you'll get 19,500 in a cold place
I've managed 20K in Keflavik in February - mind you we also dumped about thirty gallons onto the ramp from the wing NACA vent. You have to take it really slow and let it settle if you want the full amount. For practical purposes I agree - 19.5K is about max.
4000NM - just like all aircraft performance figures it's calculated from performance data and proven once during Flight Test - it does have 4000NM, but only under "standard" conditions.