The fourth 120mm is on the floor, next to the PSU, several people have setup 120 rads there.
I'll have to take a look later, but I don't quite see where one fits. Once a PSU is installed, it's cables are going to dominate that area. Or by "floor", do you mean the section that separates the main portion of the chassis with the PSU area?
I just can't picture one where you mention there's one, because there's a mesh portion that covers the entire bottom of the chassis, and any installed fan down there is not going to have good airflow at all.