2,639,456 to the nearest ten thousand would b closer to 2,640,000. This is because the ten thousands place is a three, but the thousands place is a nine, which is closer to ten. The way I think of it is the zero takes the nine's place in the thousands place, and the one adds on to three, making it four.
Ever heard of the rule, "Five or more, raise the score, four or less, give it a rest"? This is that kind of situation.