The sci-fi drama, set in the 1980s, returned in late May with a first volume of episodes showing a new supernatural horror emerging from the Upside Down alternate dimension and besetting the fictional Indiana town of Hawkins.
Fans have lavished praise on the new season, which four weeks after its release remains in the top spot on Netflix's English TV List with 102.26 million hours viewed, according to the streaming platform.
The last two episodes, with a finale running at more than two hours, are released on July 1.
"If you've enjoyed season four up until this point, get ready because the (last two episodes) are the best thing that makes what you've seen look like chump change compared to it," Harbour, who plays fan favourite character Hopper, said.
"It's a masterpiece, it's beautiful, it's epic... but it's also on a scale and a scope that almost gets silly and it's like wondrous, almost manga-esque... You're going to be blown away."
Harbour, who said season five would start filming next year, is currently in London for new play "Mad House", a dark comedy part inspired by his own experiences with mental illness.
Like his young co-stars, "Stranger Things" has catapulted him into the global spotlight since premiering in 2016.
"It's a zeitgeist. I've never been a part of anything like this but it has been seven years of this, so I've gotten used to what it means and my place in it," he said.
"It's very gratifying to be a part of something that's touched...so many young people...I always thought I'd maybe have some career of art films that would maybe appeal to old people and suddenly...I can't walk past a middle school without being harassed so it's a very strange thing for me to be a part of."