say a grand with hotel and lift tickets alone....(for 4 days and you dont get anyone to split the room)
Then you gotta factor in rentals of the gear unless you have all that
Then a car rental to get out to Vail.
Some people say they dont factor in food cause they are going to eat when they are home but i usually eat out more when traveling...plus there is the ever so obvious beer purchases so you might want to factor in those as well
id go for over 1k for 4 days unless you dont drink and have a car/some gear/friends to cut costs