I'll second checking into the break points.

If changing 16 to 12 doesn't change the rate, I would book the 16 and stop at 12 if necessary. That way you're certain to have the room if you need it, and you'll get the long term rate on all days regardless whether you stay 12 or 16, whereas having a 4-day booking would probably not get you the long term rates. Of course, check the break points.

HTSC