the best way to turn you're 2btc to 4 btc is to find some investment for bitcoin, just find the safest investment because most of the investment now are scam, or the more risky option invest in a dice site like prcdice, You will earn by investing in bankroll on site and you will earn on average 1% of all that is bet against it, however short turn fluctuations might send you in the red. Estimated Return/day: 0.2-0.01 btc ,