I have a file from TP2 that when I load it into TP4 the terrain doesn't show up in the preview/render. I rebuilt the scene using the same settings in TP4 and the terrain still didn't appear. I think that negative displacement is causing it. In Power fractals any negative displacement amplitude causes the terrain to dissapear. I am also having the same problem when I make a group of layers child layers to a surface layer.
I believe this may be an example of an issue we're aware of. Is your terrain a heightfield or procedural?
It is a Procedural terrain. I figured out the part of making the layers child layers. I connected one of the outputs to the wrong node in the node network. I also figured out a workaround for the negative displacement until the issue gets fixed. I take the power fractal and make it the color function and displacement function of a surface layer and then I can use negative displacement in the surface layer.