Interesting. I take it the 400 is still so short the main differences are still made anaerobically? Aren't the runners with the highest VO2 max like 1500m runners or so?Yes, there'se a tradeoff between those distances but I don't think anyone thinks about losing weight specifically for a change of event, I think it's more a result in change of training then of specifically thinking about losing weight first and then thinking of the training needed to the event.
400m is a border event as it can be disputed by two types of runners: the ones with sprinter builds and the progression is normally from 100 to 200 to 400 as they build endurance (not great change of body weight, I did this progression and was way faster at 400m - and the other two distances - with 75kg then when I had 70) and the one with skinny builds where normally they start in lower distances and go up with age, usually when natural speed and fast twitching fibers start decaying. The training gets you there, if you train for 800m you have to put more long distance intervals (thinking of 1k or more fast repetitions more frequently - as a sprinter the most I do is about 600m at low pace in the preseason with the bulk of the work mostly in the 200m-300m zone) than if you train for 400 and if you do that at the level required you will lose top speed.
In the same event you can have runners with very low endurance for long efforts (I have a PB of 50s in 400m and can't sustain more than 5 to 8 minutes running at a pace around 4'00/km) and with high endurance (I race against guys with the same PB who can do runs of 30 or more minutes at that pace).