Asking what fly rod to use out west is like asking what rifle should I use out west. The answer is "well it depends".
I own 3wts thru 11wts with multiple lengths and multiple actions for many of them, and all of them get used for different conditions.
Now I have taken a few trips out west to fish and my next trip I will only take two rods assuming trout will be the target, an 8'-6" 4wt and a 9'-0" 6wt. From my experience I can fish most streams with that combination. Now if it is streamer time on the Madison River when I go I will add a 9'-0" 7wt to the mix.
A 4wt will do most things needed on a trout stream and that is why you see it recommended by so many. The 4wt will handle the tailwaters, spring creeks and freestones with small size 8 streamers to size 24 midge patterns. Hopper season is where the 6wt shines.
