Those episodes describe the story between Medusa and the Ice Emperor. It can be watched without having watched the previous seasons. HOWEVER, this season is not canon at all. The events described do not appear in the original novel and will have 0 impact on the future story unless they change the story, which I hope they will not do. You can watch if you want, the art is a lot better than previous seasons. But I cannot rate higher a story that has no relation to the main story and no basis behind it. If you do skip it, you won't lose anything.