You may want to look into something a little bigger then a 4wt. I would at least use a 5wt or a 6wt. It can get rather windy in Montana during the day and it can be kind of a pain throwing flies with a 4wt.
"Unless one can enjoy himself fishing with the fly, even when his efforts are unrewarded, he loses much real pleasure. More than half the intense enjoyment of fly-fishing is derived from the beautiful surroundings, the satisfaction felt from being in the open air, the new lease of life secured thereby, and the many, many pleasant recollections of all one has seen, heard and done." - Charles F. Orvis