I think you may be looking for #8 in the diagram (since 4 are listed as quantity...and it connects the shield and the brake ducts on the (left/right) sides.
You could always take one out, then go to your local BMW dealer's part dept with the part nbrs for #5/#8/#13...then compare them to see which one is which. They don't seem to cost much.
To the far right of #8 in the part nbr index is a clickable link to see a pic of the screw...if it's not #8...then the other screws for this area if #5 & #13 (all hardware part nbrs are encircled in the diagram and usually are also listed in a separate hardware illustration within the diagram (as in this case):