Re

ROW($A$4:$A$19)-ROW($A$4)+1

Using this technique guarantees the row numbers (indices) will start at 1 and we don't have to remember whether to subtract 3 or 4 ....

so row(4) - row(4)+1 = 1 , row(5) - row(4) + 1 = 2 etc

The ROW($a$4) is absolute so remains constant.