I was looking at this with some light heartedness, however, now you mention it, a 747 is 400tonnes MAUW, so a 4000kg discrepancy is 10% of your total
Only 1%.
However, a couple of fatties in a spamcan could put it say 10% over MTOW, or more to the point 10% over the weight you thought it was, which is at least 20% extra runway length required.