trading is the best way to do it. but it requires a bit of patience is also a good strategy.
in fact you can do it on sports gambling, just pick the right match and really going to win.
but I can not guarantee, but I ever did it with a small amount.
But trading require a high level of patience and knowledge to make doubled your bitcoin.
In this case 2 btc to 4 btc , I can estimate you can make it in around two months or so.
The faster you want make it the more risk you must take. Just don't gamble it if you can afford to lose.