I suggest adding more pallets. I can't say how many would be ideal, but I put 7 stacks (of 3 or 4 each) in my 1/4 acre pond and that was certainly enough for the first year with only FHM's in the pond. Source some more pallets or at least spread the 4 you have out as single pallets held up off the bottom with cinder blocks. A tip to stacking the pallets is to put some bricks in between each pallet in the stack to create an additional layer of wood to utilize.
I'm going to guess that an egg laying FHM would like to have at least 20 square inches to "do their thing", so I doubt you can have too many structures, but you can have too little.
