…and I’d doubt it too. Were there not changes to the wing between the classic and the 400? Does this affect landing (or approach) speed?
That landing distance for different jets is typically similar if they are landing at the same SPEED (not weight) is no coincidence.
And is in fact quite a simple concept.
With the PRIMARY retarding force being brakes (in this case), the decelerating force from them F = mu * Weight, where mu is the coefficient of friction between the landing surface and the tires, say .6 or so for a dry hard surface runway. A heavier aircraft means more weight to slow down, but also the decelerating force (mu * Weight) is proportionally higher. Ergo, an aircraft with twice the weight will have twice the decelerating force, resulting in approximately the same landing distance as long as the landing speed is the same.
As indicated earlier, other factors can come into play when a landing distance is determined. Lift dump systems not only destroy lift to allow the aircrafts weight on wheels to increase, but also can provide an additional down force, increasing the apparent weight of the aircraft, without affecting the mass, and so increased deceleration is possible. Profile drag. Thrust reversers. Energy capacity of the brakes. Degraded hydraulics. Antiskid issues. Etc. But the OP wasn’t addressing these considerations in his post question.
The manufacturer has already decided how best to equip his aircraft. Thus any issue of number of wheels, number of brakes, size of tires, tire pressure, etc has already been analyzed, along with the many trade offs involved in aircraft production. If Boeing or Airbus simply got it wrong, the competition would eat them up. In any case, what is doubling the number of wheels/tires on a 747 from 18 to 36 going to accomplish? Now each wheel has half as much apparent weight on it, with those that do brake producing half as much decelerating force.
Comparing the landing distances of the 747 400 and classic “at the same weight”, as you suggested, would only add support if they are landing at the same speeds. Not being informed on the 747 series, I’ll not comment on any different wing aerodynamics that may or may not affect landing speeds.
It would be interesting to hear from ppruners based on their aircraft type what the landing speed (or Vref, or Vsr, or appropriate term) is, and the published landing distance. One could appreciate then just how accurate this relationship is.