PVC feeder can be a death trap!
Sorry to hear that happened. Perhaps that one was a tad smaller than the others and could fit in places the other couldn't. I once found a full grown chicken who had fallen behind a heavy wooden box and was stuck there. She was still alive, but I was simply lucky that I found her there, because I would normally not look in that place.
Did you mean to say 4-inch pipe instead of 4-foot? If so, then that would be helpful info for others.
Another common mistake is to have deep water (deeper than they can lift their heads above to breathe) with vertical walls. If there is a ramp they can walk out on, they might make it out, but if it's vertical sides (like a kiddie pool, or any pool for the matter), they will drown.