Well, EP Shawn Levy was recently asked that very question and answered with the following:
“Well, we never wanted to run on fumes and we wanted to only tell as much story as the brothers saw and feel with clarity. So it’s been clear for a while that they know exactly where we’re headed and this is the arc. We wouldn’t want to stick around for one moment beyond that vision and the clarity of that voice. So rather than focusing on that sad day that happens who knows when, the end of season five, I’ll just once again reaffirm what I’ve told everyone, which is it has been a long-ass wait for season four. But I swear to God, the world will realize it’s worth it.”
Look for the first half of Stranger Things 4 to arrive on May 27th, while Part 2 of the run will then debut on July 1st.