I looked at the issue and it seems that the problem is a regression from https://github.com/Slicer/Slicer/commit/a22c49cab51666a7a1c2a0ddcf79d329a4da08c9

The local variable modelDisplayNode has been partially renamed into mrmlDisplayNode. However the lines after L1557 have not been renamed.
https://github.com/Slicer/Slicer/commit/a22c49cab51666a7a1c2a0ddcf79d329a4da08c9#diff-935cfa1ad640cc957cd6fb1519cc08dbL1557

Not being familiar with that code, I would prefer to let Nicole comment on what's the best way to fix it. (my naive thought would be to rename in SetModelDisplayProperty after line 1557 all the modelDisplayNode into mrmlDisplayNode ).

I was trying to separate out the operations that were specifically defined on the model display node for the scalar settings, and saved the hierarchy display node in line 1492 (in the github version Julien linked) to the more general mrml display node pointer since it was obtained as a mrml display node, not a model display node.
Will take a closer look at how the hierarchy display node is used and fix it.

Cast the model hierarchy display node to a model display node and use it for determining model colours if the hierarchy's display settings are to over ride the model's display settings.
http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=23549