Could be as much as fleas and that's where they can bite at them (worm them fleas lead to worms). Or it could be something that's catching ,hitting bumping or pulling on the hair on the tail which with 4 the same time and place makes since. Look around feeder water bucket, dog house, regular areas that all 4 pups would go. I had a couple Hounds that the hair on there back above there shoulders big circle areas of missing hair, treated for every thing turned out hound had chewed a corner on the dog house and the edge was jagged every time the dogs came out they would hit that corner and pull the hair out. One other could be the pups playing chasing fighting biting. Hope this helps usually just needs a little looking into.

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