Node placement

I am looking to create a dependency graph that flows from left to right. however I want to place nodes in specific vertical columns.
so e.g.
node1->node2->node3->node4;
node1_1->node4_1;

if these are the dependencies, I want node1 and node1-1 in one level (column), nothing under node2 and node 3 columns and node4_1 under node4. That way I can visualize a picture something like:

node1----node2----node3----node4
node1-1--------------------node4_1

Any suggestions?

Use rankdir=LR and rank=same

Use rankdir=LR and rank=same to set up the column constraints:

digraph G {

  rankdir=LR

  node1->node2->node3->node4

  node1_1->node4_1;  

  {rank=same node1 node1_1}

  {rank=same node4 node4_1}

}