As far as I remember, 400ft AGL is because 300 feet of obstacle clearance (The same that circling approaches). Because you can't think no obstacle exist (a house for example) 300ft + Obstacle (not high because of runway heading) rounded 100ft above will give you 400 ft. That is the minimum height to perform action safely in the very close vicinity of your airport.
If you need to accelerate because of engine failure, your accelerate height can be circling approaches minimum if VMC or HSD if IMC.