We have dotted lines painted on the road for this, as we actually have a similar situation here on a road I am on all the time. 4-lane road, lanes 1 & 2 are turning left onto a 4-lane road, lanes 3 & 4 continue straight. However, contrary to what @Xmetal said, here, because of the dotted lines that guide you thru the turn, turn lane 1 cars can enter into lanes 1 or 2 of the new road, and turn lane 2 can enter lanes 3 or 4.
They have dotted lines on many of the roads here to help drivers through turns. Mostly because I live in Ohio and the drivers here are stupid as hell (I'm from MI
).