Upper body and core strength is hugely important in the 400 meters. By the time you hit the home straight you should feel like your legs are going to give out. At that point you rely on your arms, pumping your elbows back with your shoulders as relaxed as possible. Your legs will follow. Keep your elbows at 90 degrees and brush your shorts as you swing your arms. Your arms should come up to eye level
Weight-wise your never going to be a LaShawn Merritt (13 stone) or Kirania James (11 stone 6) but Olympic Silver medalist Lugelin Santos is 9 stone 10, so you may want to do some research in to his regimen.
You do need to increase your weight without increasing body fat. Increase your protein. Eat lean, chicken breast instead of legs, lean red meats, ask your Mum to get the 95% lean mince. Eat fish, without batter. Get some good fats, tuna, salmon, olive oil and nutsare ideal. Don't use salad dressings, use a tablespoon of olive oil instead. Eat crisps and chips less less often. Scrambled egg-whites are better than fried eggs. Drink skim milk. Minimize your intake of soft drinks and beer. Get extra portions of veggies.
If you don't have access to weights then you can still weight-train. The are free apps on Google Play and Apple Store that will help you increase your max number of press-ups
https://play.google.com/store/apps/details?id=com.rittr.pushups&hl=en
. If you can't do a full pull-up kneel on a chair. You can do front and side shoulder raises by filling 2 gallon jugs with water.
Core, hips and glutes. There are plenty of body weight exercises. You may want to consider a rubber band with a felt pad that goes under the door
http://store.gofit.net/GoFit-Tube-Band-Door-Anchor-p/gf-stda.htm
For glute and exercises you can lean on your elbows, stretching a leg out straight and doing large circles, clenching your bum. You can add back and side kicks, side leg raises and crabs using rubber resistance bands. exrx.net is a great resource.
For core, obviously front and side planks
https://play.google.com/store/apps/details?id=com.plankathon2&hl=en
, half crunches (to protect your back), bicycles, leg raises and lots more.
Flexibility is important. Michael Johnson says his performances improved after adding stretching to his regime.
Mental attitude. You can decide what is best for you, being relaxed and chatty before a race or focusing 100% on what you want to do.
Michael Johnson's race tactics. Go out hard over the first 50. Cruise the next 150 focusing on turnover. When you hit 200, refocus and run the third quarter as if it's your last. Run on the inside of your lane. When you hit the home straight say a prayer, even if you are an atheist and rely on pumping those arms.
Race rehearsal. Run 250s an 300s using the above race tactics. I assume you use spikes and blocks.
Run hard workouts on successive days, to ensure you get used to working out tired.
Long runs aren't that important. 40 minutes should be enough.
All the above is based on research I've been doing over the last 12 months. Others feel free to disagree. I suggest you do the same and find out what suits you.
I like this guy's site
http://www.brianmac.co.uk/sprints/tp400.htm