Sodium polysulfide is a mixture of sulfides. In the mixture the amount of each polysulfide decreases as n increases. I would make up a 30% solution of sodium sulfide and keep dissolving sulfur in it until no more will dissolve.
The structure consists of a chain of sulfur atoms with a sodium at each end. For Na2S4 this would be Na-S-S-S-S-Na. You can have a chain of perhaps a maximum of 6 sulfur atoms. The exact number varies depending on what text you read.