In theory, you might be able to achieve something like this using an unbound column and a CreationFilter. I say "in theory" because I have not tried this in the tree, but I have done it in the WinGrid and they both use a GridBagLayout to position the cells. 

The idea is... in a GridBagLayout, it's possible for two objects to occupy the same space in the layout. The designer won't allow this, but you could set it up that way in code.

So what you do is add an unbound column to the ColumnSet and set up the column's NodeLayoutInfo so that the unbound column spans across the entire node. So in the example you have here, assuming each column has a SpanX of 1 and that the RootNode Column has an OriginX of 0, then unbound column's settings would be:

OriginX = 0

SpanX = 4

So the unbound column occupies the same same in the layout that the 4 other columns combined occupy.

Then, for each node, you either have to hide the unbound column or hide the other 4 columns so that only one or the other shows up in each node. In the WinGrid, this can be done using the hidden property on the cell. The tree doesn't have a Hidden property on the cell, so you would have to hide the UIElements using a CreationFilter, instead.