The 4th goes on the hive body along the front opening.
We also have the Beetle Baffles mounted on Weatherproofed Spacers, ready to go on your hive here
From the inventor:
'Although they can fly very adroitly, SHBs don’t often fly inside the crowded hive. It is likely the limited airspace and their hesitancy to conspicuously expose themselves that cause them to usually crawl from place to place.
When beetles enter the hive entrance each beetle can move as they see fit barring some intervention of the resident bees. Most beetles we observed seemed very shy when first entering the hive and stayed in or very near vertical or horizontal corners. As the beetles became more acclimated to the hive environment they began to gradually move slowly in all directions but those moving upward were stopped by the barrier. When the beetles moved up to the barrier level they moved laterally, downward or just stopped and huddled below the baffle. Eventually the beetles began to crawl down to the solid bottom board we used for the experiment and sought out every possible crack or crevice that might make a hiding place.
Based on our observations we believe that a well-designed screened bottom board trap pan would have seen lots of business that day.
The Beetle Baffle barrier system cannot completely prevent each and every invading SHB from reaching the sensitive parts of the hive. It is, however, very effective at eliminating otherwise convenient access for the SHB. Limiting the vertical access to the beetle forces them to spend more time looking elsewhere for a good place to hide where there are sources of food, other beetles to breed with, and places to lay eggs. We believe the Beetle Baffle in combination with a screened bottom board and trap arrangement is a killer combination…..literally!'