Ah the question old as time. Best answer is when it's ready. Many of us would like to know but the problem is it's all volunteer work. We just gotta be patient. Or you could probs donate to the devs to keep the awesome work moving along. They would probably be able to work more if they didn't need to have to worry about us all the time either. Long story short, no one knows
PocketMine 4.0.0 API is building. For more information: https://github.com/pmmp/PocketMine-MP/blob/stable/BUILDING.md