You are hunting with a guide????? Most likely.
He will let you know the difference. If not, a 360 elk is definetly huge but 390 or 400 inch elk, there will be no question or doubt about it in your mind. You will know in an instant if you have a 400 class bull in front of you.
Basically his rack will extend from his nose to his rump and will be as deep as his body is from his back to the bottom of his chest.
I sure as heck would not pass on a good 360 inch bull no matter if I were hunting in a "trophy" unit and could possibly have the chance at a 400 inch bull.
Go here if you want to see the difference between 360 and 400 inch bulls. Scroll down to the bottom and click on the "350 inch bulls" or "400 inch bulls".
http://www.kingsoutdoorworld.com/trophy_room.htm