Excel refers to the wrong cells

[Thibalt]

Hi, I have a question.
I`m trying to make a column wich refers to other values in another column, but when I try to extend the column (with the black cross at the bottom right). It gives mee the wrong values.

Example: D is the column that I tried to make. I typed the first three values in column D en then I extended it. But instead of giving me A13 A17 A21 it gives me A4 A8 A12. How can I fix this?
....A....B....C....D....E....F....G....H
1 X_________=A1
2 Y_________=A5
3 Y_________=A9
4 Y_________=A4
5 X_________ =A8
6 Y_________=A12
7 Y
8 Y
9 X
10 Y
11 Y
12 Y
13 X
14 Y
15 Y
16 Y

[DonkeyOte]

use INDEX

D1: =INDEX(A:A;1+4*(ROWS(D$1:D1)-1))
copied down

[Thibalt]

Thanks!