There was a time when people thought that Stranger Things would only last up to three seasons, but because of the enthusiasm of the people who watched the show as well as the actors of the series, they have decided that they will continue up to one more season.
The season has been renewed for season 4. People even say that they will probably have one more season after that. It will not be surprising if the series last for up to five seasons. After that, they would decide if the series would continue or not. Fans of the series will probably miss it once it stops airing.
There is no confirming or denying if there would be a season 4 of Stranger Things. However, from the season 3, we can tell that there would be a season 4 because of many unanswered questions in season 3. Ross Duffer once reveals that season 4 will be very different from the past seasons.
This is a hint that season 4 is coming. The Duffer Brothers also mentioned that Stranger Things is likely to end after Season 4 or season 5. Therefore, we should expect a new season.