colder temp was will be harder, because at those cold temps, the snow is harder. i dont understand why a company would have a 0 to -30 wax and a 0 to -4 wax tho. just get whatever temp range you ski in the most.
Snow crystals have different structures at different temperatures. In warm conditions, snow crystals are softer and wax mainly serves to break up water tension underneath the ski. Without wax (and proper base structure for warm snow) you would literally stick to the snow. In cold conditions, snow crystals are very sharp and hard and dig into the base of the ski slowing you down. Cold wax is harder and helps form protection against the sharp snow crystals from digging into your base.