Although the question is how to lose 400 calories on a treadmill, it would be pointless doing that if you didn't have a weight loss strategy as well. What other reason do you have for wanting to burn 400 calories. Just burning 400 calories per day and eating the same amount of food to maintain your weight would mean you have a 400 calorie deficit. This, in time would help you lose weight.
To lose 1 pound of weight you have to burn 3,500 calories per week. That's 500 per day. So the 400 calories make a big step to your weight loss goal. All you would have to do is eat 100 less calories than you were before to start losing around a pound per week. Combined with the weight loss due to the calorie burning you will find inches will come off your waist and thighs.
Remember, every body type is different so for men if you have that stubborn belly that won't go, it may take a while to see it shift. Women if you have a naturally bigger butt, then that could be the last place that you see the inches shift. Embrace how you look and weigh yourself weekly at the same time of day. This way you'll see predictable results from your workout efforts.
Take it easy and take rest days
Burning calories is the easy part. The hard part is the patience. We can all burn 400 calories but sticking to that routine for weeks or months at a time to see results takes commitment. Run for 3 times a week or more and if you do more take 2 rest days a week. Losing weight doesn't happen overnight but it does happen and your calorie burning is what does it.
Even when you rest you're still burning calories at a higher rate than if you had not done the exercise. Athletes have rest days to help their legs heal from the work and this is the same for you. Even if you're walking, which still counts as moderate exercise, take rest days. Then enjoy people commenting on your weight loss because of your workouts on the treadmill.