I’m not sure what Class 3 & 4 fit means. I have always preferred aviator style sunglasses for high altitude, snow-covered and sunny terrain. You can easily get prescription lenses for this style of sunglasses. You can even get them with bifocal lenses: I did. If going for bifocals I recommend the reading part be set 2mm lower than is the norm. That way the reading part won’t interfere as much with your view of the trail.