@Kadri - very strange
@Dune - It looks like the cube you're using originated within Terragen, e.g. it wasn't imported using the obj reader. I tried the same, just setting render to invisible and leaving cast shadows checked, and got the same results as you. Seems like it's just the obj reader that's giving me trouble.
I did a new test, exported a new cube from maya, imported it into terragen with the obj reader. It seems like even when I change the cube's settings, nothing updates in the actual render (see attached).
At a loss... I feel like there's just some setting ticked somewhere that is doing this, or i'm missing something really obvious... Sort of hard to troubleshoot if it's working for everyone else. Using the pro version of tg4, btw.