Hi Chris,
Do you have any examples you can post?
For hand held 400mm you would need a fast shutter speed to catch a moving subject so set the mode to TV, and a speed of 1/1000. If that is not giving you enough light push the ISO up to 400, before bringing the shutter speed down.
Put the IS on (IS mode = 1). If you are panning, set the IS mode to 2. If you are shooting into the sky, make an exposure compensation of +2/3 stop.
I hope that helps for starters.
Good luck.
Col