This was revealed by the show’s producer and director Shawn Levy who spoke to the folks at Collider. According to Levy, “I mean I will say we have a good sense of stuff that happens in Season Four. Season Four is definitely happening. There’s very much the possibility of a season beyond that one, that’s currently undecided.”
It is interesting that a fourth and possibly fifth season has been ordered when it is unclear as to how the third season will hold up. Not to mention the third season is only expected to arrive in 2019, and also the fact that the show was only supposed to be a limited series where one of the show’s protagonists, Eleven, was originally planned to be killed off.
In any case only time will tell if the show will warrant a fourth and fifth season, after all there have been plenty of shows that have gone on season after season but not necessarily for the better, and hopefully Stranger Things won’t be one of them.