Definitely be prepared for snow and for very rapid changes in the weather. I haven't hunted there in years, but was there years ago. On May 8th had a big snow dump and then turned very cold. Be prepared for anything. Have had the same thing happen just north in Carter county MT. It had snowed a bunch in early May. It then turned warm and the roads turned to some very slippery gumbo. Definitely have 4x4. My son lives in western WY, but hunted the Black Hills of WY the opening weekend two years ago and it was fine. If I were you I would personally go later than 4/20, but even later be prepared for anything and it can change in hours.