@ImDanGrow Each soul land story is set 10,000 years apart from each other (if I remember correctly. It may 1,000 years) But each soul land spoils the others. For example in soul land 2 you find out the ending of soul land 1 and what the main character did. Same goes for soul land 3 and 2. It may be easier to just read the novel. But if you don’t want to read the novel and don’t care too much about major plot spoilers and endings then read the rest. (Personally soul land 2 is my favorite).
At the point where we are in the 1 we know how it'll end in the grand scheme of things. And the links between SL2-3-4 are way more blurry than between 1-2, you should be able to read it without much problems. At least with what has been shown in those at the moment, later we'll probably get some huge spoilers.