If you could hack into the parameter settings of the P4, you'll find the wind warning is set to "6", presumably meters per second, which is about 13mph. This estimation is done with craft angle, measured with the IMU, which isn't an accurate way to measure wind speed, hence the annoying warnings. The most accurate way to check wind speed at altitude is to flip the craft into ATTI, give no stick input, and watch it drift with the wind. Watch the MPH speed in Go increase. When it stops increasing, there's your actual wind speed. Usually it takes about 20-30 seconds of drifting to determine actual wind speed.