Option drag your object horizontally to make a copy or use Move and copy if you need a precise increment. The hit Cmd+D a few times to make duplicates with the same spacing.
Hit Enter again and make a note of the increment on a piece of paper. Cancel.
Select everything and use the same increment, now in a vertical direction, to make 2 further copies.
Take the middle row and move it horizontally by half of the increment. This means that the circles in the middle row are positioned horizontally half way between those in the other two rows.
Draw your defining rectangle so that its corners are on the centres of those 4 circles. (This gives you its right size but you may possibly need to move it a bit.) Send it to the back of your stack.
Drag those 8 circles and the rectangle to your Swatches panel.
(If you’re on Windows read Ctrl instead of Cmd and Alt instead of Option.)