My guess would be they�re probably just hiding. It�s summer and hot, and your pond is fairly deep. The only way you will know if they are in there is from fishing, so it might be worth trying to fish and see what you catch. It is unlikely all 3 died from old age at the same time, and if they died from anything it is likely you would find bodies. Also the presence of the 4th bass indicates water quality problems that would kill bass are unlikely. Bass are also somewhat terrortial and in my pond with only 5 large bass, they seem to have their own terrorities and if you see one bass you never see or catch a different one in the same area. Since your pond is fairly small surface area wise, I would just try fishing in the early mornings and late evenings and see what you catch. The one problem is bass get very stressed from being handled in hot weather, so there�s that.