One would imagine Netflix will give the Duffer Brothers the budget for however many episodes they’d need, so it really just depends on how many hours it will take to effectively tell Season 4’s story.
Stranger Things may announce a 10 episode run for Season 4, or maybe even go with 6 or 7.
It seems they have a sweet spot in the 8 to 9 episode range, so I really don’t think that’s a wild assumption to make for Season 4’s episode count.
Of course, just because the sun rose today doesn’t mean it will rise tomorrow.