Do NOT make that decision strategically. Have sex with him when YOU want to, not when you think it’ll send the right message. The right message is the accurate one: the #1 lesson (and sometimes only) info he’s going to take from it is how much you like sex and how much you like him. Both of those are things he needs accurate information about How many dates have you been on with him? Do you WANT to fuck him now?
If he’s been on four dates with you, YES he wants to fuck you. Probably even more than you do. Whichever one he thought was “the I’m not a slut date”, you’re well past it. You’re torturing yourself bc you’re nervous but you don’t need to be. Call that guy up and nail him before his blue balls start dragging on the ground lmao, he’s probably got em taped to his calf by now