I still use Terragen 3.3 so not sure if all i say below is true.
I tried your file. Not quite sure but it looks like there is some funky thing going on.
If you put the displacement node in the "Mesh displacer" input of the boulder object you will get displacement.
But for the population to accept the displacement you have to check the "Flip normals" box 2 times.
This is what i used at least. There might be other ways too (see below).
It is like if Terragen can't refresh the object to the new displacement.
Not sure if this a problem related to your object.
You can look at the files below.
I put 3 example files in the zip.
To see what i try to say better try this:
* Open the "Boulder test_03.tgd" file and make a render.
* Now open the "Mesh_displacement_Power fractal shader" and change the displacement amplitude to "3" and render ones again.
There will be no change. It will render the same.
* Then disable and enable the "BoulderV1.obj" object node and render ones again. The render changes only now (a smaller more
erratic displaced object with correct population).
To see the new displacement you need to kind of manually refresh the object.
Not sure if this is your problem but there is a bug probably.
