If you don't need the power changing to a 5000 mAh 4s would make sense.
You would be loosing 20% voltage so rather more than that in maximum power but you are also saving 20% battery weight so you won't need as much power to achieve the same performance.
With he reduced voltage you will also be drawing less current so you could fit a bigger prop (either diameter or pitch) to restore the desired performance. .
The only practical way is to try it and see.
This sort of experimenting is very interesting but I would strongly recommend you get a Watt meter so you can measure the actual current to make sure you are not over loading the motor, ESC and battery..