Looking over the comment and the wiki sections I was wrong with the change to 4 but I think maybe there should be a lane tag "turn:lanes:forward; ||merge_to_left" to account for the lane merging in the forward direction or maybe that tag should be in the next leg of the road. (or both) 

Let me know if I am misunderstanding the turn:lane tag in this situation. I looked at the wiki as you recommended for it. 