4 Players.

[Judtech]

Had this exact scenario today. At midcourt, ball on baseline back court. I had subs coming in, and hand the "Stop Sign" out for my partner. As I am counting the players, I see confusion at the bench and only 4 players on the court. I keep my stop sign up, and double check to see if I hadn't missed anyone. I don't see my partner put the ball in, but I IMMEDIATELY blew the whistle and killed it. Since my hand was still clearly up I didn't want to have someone run onto the court and then we get into a furball T.
All that to say, if no one notices there are only 4 until the ball is in play then that team plays with 4 until the next dead ball when they can legally sub