4s would be 12 volts. 9V is for 3S. You don't have to discharge after every flight, but it may be a good idea after a short flight, or periodically like every 10th flight or something. But if you are flying until around 12V it really isn't necessary. Discharging to only 3.2V per cell may be better for the battery, with a periodic discharge to 3 per cell.
The charge rate depends on the battery, which should come with a charging C rating. Generally you charge at 1C to get the most life out of the battery which takes 1 hour. Charging at 2C is pretty common which takes a half hour. You can use a charging board to charge multiple batteries at once.