Tom’s IMDb profile lists him for just one episode in Season 4, so it’s unclear how big his role will be. It’s possible that he will be in more episodes, especially since, judging by his quick appearance in the Season 4 teaser trailer, he plays an evil Russian soldier. Right now, it looks like he plays a soldier who has an eye for torture and Stranger Things really wouldn’t be an authentic 80’s TV show without evil Russians, one of which Tom will play, come Season 4.
Source : https://www.distractify.com/p/stranger-things-season-4-jaqen-hghar