The WR for long course 400m freestyle is almost identical to the mile WR. More swimmers are able to achieve this mark but the training intensity needed is similar. In swimming, fewer athletes are limited by their body's inability to withstand brutal workloads without being injured so more athletes develop their fitness to the highest level possible. I know several talented runners who, had they been able to handle truly elite workloads, without succumbing to injury, would have definitely given sub-4 a scare.
15 min 1500m for men.