Yeah there are 16 spots available on the stock screen. It is 4x4. The reason for this is because when you rotate it to the side, the icons stay in the same order so it has to be a square. In order to make a 5th row, it would have to be 5x5 which would make them tiny to fit the short side of the screen.
Its kinda hard to explain.
Imagine 4x5 like you guys want and you have it setup like so:
I I I I
C C C C
O O O O
N N N N
(each letter is an icon/app/single widget)
now rotate that on its side and keep the SAME layout.
[4x1 widget] (there is now an extra space NEXT to this widget..)
I I I I (another extra space here)
C C C C (another)
O O O O (another)
and you can't fit the N row because it is only 4 tall now.
So to keep everything in the same order it needs to be a square grid.