I've been a competitive swimmer all my life (well from 6-18, i'm 20 now) and the 400m swim is all about technique. My best advice is to work on your technique with a buddy who swims or find a good swim coach and the 400m should be cake. I've seen countless people do the 400m swim for DM and just lose it because they are expending too much energy. Surface swimming can be compared to diving. You get better air consumption when you have good technique (buoyancy control, no arms flailing, etc). Swimming is way easier and you use less energy when you have a proper technique. Work on your technique and work up a little to the swim and you'll do fine. My old 400 yard time (I swim yrds not meters, sorry) was about 4:10 when I was in good shape. After two years of not swimming really at all and not working out much I jumped in the pool and did my DM swim in 6:02. I had put on about 15 pounds and my stamina was horrible. The only way I got the time I did was because my technique was still there.
-Kstnbike
-Kstnbike