If you were to head the columns from right to left they go something like....1, 2, 4, 8, 16, etc etc.
Therefore: -
0000 = 0
0001 = 1
0010 = 2
0011 = 3
0100 = 4
0101 = 5
etc
As you can see, there's no real need for the leading zero's as they mean nothing, but I added them so you can see how it works. Simple really. 馃槃