First, lower your Atmosphere Samples. 128 is really high, *especially* if Defer Atmosphere is enabled (which it is by default in Terragen 4). Just leave the atmosphere at 16 samples until you know you need more (when you see noise in the atmosphere). Higher Atmosphere samples will *not* give you sharper results.
Second, keep in mind that many of the renders in the Terragen gallery, and that are shared here, have some amount of *post processing* applied to them. Often it is minor, just sharpening for example, or a little downsampling, and maybe some minor contrast or color adjustment. But this can definitely give a nicer-looking result than a straight, unedited render.
Finally, you might consider changing the Pixel Filter that is used with the Antialiasing (in Renderer settings). You can try a "sharper" filter, they are mostly labeled by what they do. You can also just experiment with them to find one you like.
Overall I do not think there is *anything* in the render settings of the images you are seeing from other people that is making their images sharper than yours. Perceived "sharpness" is often much more to do with the construction of the scene, and the types of objects, textures, etc. that are in it, as well as subtle differences in post processing (e.g. Unsharp Mask). Testing on such simple terrain-only scenes is also probably not going to give you much information. Terrain is, in general, not very "sharp".
- Oshyan