Exporting project assets from Terragen has come up a few times on the forum, so here is a summary of some things to be aware of.
In Terragen 4 Professional, you can export cameras, lights and null objects via the File menu > Export FBX scene. This option does not export terrain.
In Terragen 4 Professional, you can export terrain via the Micro Exporter feature found under the Renderer > Sequence/Output tab. The Micro Exporter saves meshes in several file formats including FBX and OBJ. The polygonal triangle mesh output of the terrain is identical between the FBX and OBJ formats.
We recommend that you set the "Maximum threads" setting to 1 for the render of the Micro Exporter. You can find this setting on the Renderer > Advanced tab. Also increase the "Max bucket size" to the same size of the image. This setting can be found by clicking on the "Bucket controls" button on the same tab and avoids having to render overlaps at the bucket borders.
After the terrain has been saved as an FBX or OBJ you should process the mesh in a third party application, such as MeshLab, to merge any duplicate vertices.
Finally, here are the links to the online documentation for both these features:
Exporting a Terragen project as an FBX file:
https://planetside.co.uk/wiki/index.php?title=FBX_Export:_SceneExporting a Terragen terrain via the Micro Exporter:
https://planetside.co.uk/wiki/index.php?title=Micro_Exporter