Have 4mm kidney stone in mid pole of left kidney. Have a long flight, India to USA in 2days. Doctor says 4 mm is nothing to worry about. What you think?

Hopefully fine. There is always a small chance that the stone could begin to move into the ureter (kidney tube) but this is not likely. You may want to consider treatment of the stone electively before another long trip or important event so that chance of an event is eliminated. Would talk to your urologist.