Okay, first things first, don’t listen to any of the replies thus far, these people are not sprinters.
Having said that, like all sprints, the deciding factor in the 400M is top speed. Unlike the shorter sprints however, the acceleration component of the 400M is limited. In the 100M you need a good start to win, but in the 400M it’s all top speed and special endurance, the contribution of the start is negligable.
Since the 400M is such a long race, being light is where it’s at. Jeremy Wariner, the world’s fastest 400M man, is 6’ tall and weighs 150 lbs. Everyone on this site would tell him to bulk immediately, but he’s built for his sport and that’s what matters. As a 400M man yourself, your focus should be on top speed (fly 30s) as well as special endurance (15-45 second runs).
As far as weights go, stick to the very basics. Squat and GHR relatively heavy, but keep the reps on the lower end. Also, remember that weights are secondary, so don’t wear yourself out in the weight room. Lift heavy, but leave plenty in the tank.
The weight room can make you a hell of a 60M sprinter, but it won’t make you a great 400M sprinter. Don’t focus on getting stronger, that will happen on it’s own, focus on getting faster.
Let me know if you have any more questions.