man, you've got a challenge here.
A 400 is like a giant tuning fork in the wind. I've photographed birds w/ a600+teledapter, and it's difficult.
Some tips:
go to Really Right Stuff and see if they have a shorter leg/mount for the tripod ring. The shorter that is, the better. Same for the ball head.
Use a carbon fiber tripod with only three sections. Keep the tripod as low as you can and still get the shot.
Some people put a small sandbag on top of the lens. Keep in mind that this WILL make the whole setup even more top-heavy than it already is.
Use a remote release or shutter delay and mirror-up. These are essential.
If I'm in a hurry I will put my hand on top of the lens, at the point of the tripod mount.
Weight the tripod underneath with a hook, or improvise.
Some people set up a second tripod to support the lens.
Last, remember that the whole rig could go over in the wind. Seen it happen in Utah. Stay close.
Hope this helps,
David Saffir
http://davidsaffir.wordpress.com