The valve lash should be in your owners manual, if you cannot find or dont have the owners manual then check on one of the stickers on your frame tubes. One of them should list most of the specs for your ATV.
As for adjusting them, if the pinned 2004 manual doesn't cover the 400 then try the 2006 as adjusting the valves would be the same process. A couple months ago Quad Magazine had a nice How-To writeup on adjusting valves, to bad I dont have a scanner.