Yeah ik the vikings only had 2. Summer and winter. Winters was the measure of years a viking has lived But id also add spring and autumn for added effect. It would be visual change mostly weather would be effected. Day time durring the spring and fall would give u the "cold" debuff summer would just be how the game is now in each biome. Winter on the other hand would make the meadows and black forrest like the mountains where u need to dress warmer or drink frost resistant mead. And evrything would be cold and wintery. The water in the swamps would be frozen over t make travel over it easier. The plains would be a bit warmer be more like autumn or spring. It would still snow sometimes and rain other times. winter cold would stack with the mountains and deep north where not only do u need to dress warm but u also need frost resistant mead or a fire or else the freezing debuff will apear.