Instead of the formula that calculates from the row above, or dragging the first few numbers to fill in the rest (which will populate those cells with constants, which may be a problem if you delete a row) you can also use this formula in each cell of the area you want to contain numbers:

=row() - 3

The row() function equals the row number of wherever it is: if it's anywhere in row 4 it equals 4, anywhere in row 5 it equals 5, and so on. It must be followed by () for arguments even though it doesn't take any. Subtract 3 if you want row 4 to contain the number 1; adjust this for however many un-numbered rows you have at the top.