I don't have experience on the North Platte, but have fished a few windy rivers and caught several 18" plus fish on a 4wt spey. An 18" fish will most certainly put a bend in a 4wt! The IMX Pro rods are really nice rods for the money and I would think you would have to go custom (Anderson, Meiser or Burkheimer) before you could really tell you were getting more rod for the money. The IMX Pro rods are pretty stout for the designated line weight so I would stick with the 4 over the 5. Anderson's rods are super smooth (I have the 1173) and one of his 5wts that throws in the 330-360 grain window would also be perfect (he was coming out with a 1205 this year that would fit the bill). Given all the room you have to work with I would go with a rod as close to 12ft as possible and try an airflo rage head (now available in the lighter trout weights) as they were specifically designed to help in the wind.