Placing your cameras in the external corners and the interior corners of the showroom is not sufficient. That's because your doors are located halfway along the 40' long wall which means someone entering/leaving the shop is likely 20' from the nearest camera. That is too far to get any usable facial features on a camera with a "normal" (ie roughly 90 degree) field of view. Of course you could mount cameras with a much more narrow field of view in those located so you could get usable facial features of a subject 20' away, but that isn't what you are suggesting. I would suggest getting some of the 180 degree cameras (which are effectively two 90 degree cameras together) and putting them in the showroom and the exterior of the building. (See my attachment and the purple half circle shapes for a rough idea of placement. Actual placement would depend largely on where the best placement is to cover the doorways. IE you probably need to cheat them towards the door). This way you "only" have four cameras in the showroom, yet they would out perform having a camera in each corner (so still four cameras) by leaps and bounds.
Furthermore, the two cameras located behind the checkout counter should be mounted on the side walls as close to the counter as possible and you might want to add another camera with a narrower focal length that is designed to only cover the entrance (the orange camera in the drawing). I had initially placed this "entrance camera" it in the upper corner behind the checkout counter to be less conspicuous, but the way the door opens on the drawing, that probably isn't an ideal location. I would also add an internal camera facing the rear door too.
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