ROWS(A$4:A4) is simply a counter... It removes the need to have that additional column of yours: 1, 2, 3, etc.

in A4, it resolves to 1, in A5, to 2, etc. I looked at canapone's first solution and didn't understand it. So, while to-ing and fro-ing to my garage, I fiddled with it so that it was understandable. We both got to more-or-less the same end point... though he was 30 mins quicker than me.