First off-

What is Caster- I know camber is the tilt in the wheel. Does caster = toe-in?

Is it adjustable on the 400ex? If so- what is some good settings for typical riding styles?

 

castor is the angle on which the suspension is compressed. if you look at your quad (profile) the a-arms and spindles are on an angle. that angle is the castor. when the suspension is compressed the wheels move up and toward the rider. straight up would be 0 degrees castor. castor also plays a major role in steering. castor and toe-in are not the same things.

I have a 300EX, so I'm not absolutly sure about the 400EX but, I dont think I've ever seen a quad come from the factory with adjustable camber or castor.

you can adjust your "toe" by playing with your tie rods. most riders prefer a little toe-in for high speed stability. I'm sure someone can post the "perfered" amount of toe-in for your bike.

 

Castor is the relationship beteen your upper balljoint and your lower balljoint, usually expressed in degrees. Positive castor means the upper balljoint is tilted towards you, negative means it is tilted away from you. Generally speaking, positive caster will give good straight line and high speed stability, while turning will suffer. Less positive castor will give you better turning, a nice light feel to the frint end, but high speeds will make the front end "twitchy" and it will want to track every small rut you come across.
Don't confuse this with the upward "rake" that the front of quads have in the frame. You will notice that the front of most quad frames tilt up where the suspension ( A-arms) attach to the frame. Honda uses about 14 degrees of rake if my memory serves me correctly. This allows the front wheels to move back slightly as they cycle through the suspension travel. This results in a much smoother suspension action than a Banshee, for instance. The Banshee only uses about 7 degrees.
Most people do not have the epuiptment or the knowledge to make castor changes, plus, Honda has it just about perfect so I would not mess with it. The only thing that changing your castor will do for you is a slight difference in your handling characteristics that most riders,(myself included) will not be able to take advantage of anyway.
Now, if your new a-arms are castor adjustable, your upper a-arms will have spherical rod-ends to attach to your frame. Stock arms are not adjustable. Your settings will be a matter of personal preference so start at a neutral setting and go from there.
Toe setting will be the same as your stock setting and camber should be slightly tilted "in" with the rider on the machine.
Good luck.