If you want to get from 10 btc to 40 btc, just bet on 4x.
That is actually the "safest" way to get to it, because you are only hit with the 1% house edge once.
If you keep betting, the 1% house edge adds up and you will slowly lose money.
If you are lucky enough to make it to 40 btc, you will do it in that bet, but there is ~75% chance you will lose.
Nothing is "good and relatively safe", its all your luck.