Drag Formula's to continue a sequence

Good morning all,

I'm trying to use the dragging ability (little black box at the bottom of a cell when selected) to continue a formula sequence, and the formula refers to another sheet.

For example:
In A1, =Dets!A20
In A5, =Dets!A21
In A9, =Dets!A22

And so on. My current issue is I can select A1 - A12 (A2-4, 6-8, & 10-12 are blank) and drag down, but my sequence keeps skipping.

For example:
A1 =Dets!A20
A5 =Dets!A21
A9 =Dets!A22
A12 =Dets!A41
A15 =Dets!A42

in a1, a5 and a9 put
=INDEX(dets!A:A,CEILING(ROW()/4,1)+19)
select a1:a12 fill down