Boeing 747-400, Why is it called that?

The 727 and 737 came out before the 747 next was the 757 then the 767 and so on. The first three digits do not actually refer to the size of the plane just that happens to be the order they came out. The 3 digits after the first 3 is what tells the size of the aircraft. The 747-400 is the largest of 4 models of the 747. The 747-200 being the smallest of the 747's, but still a 747.