Rich, the User Interface will be similar to Terragen 2 and 3, but we're working on some changes that would make it easier to use (simpler methods for editing some things, faster preview to get better feedback on your changes, etc.). Documentation is always a work-in-progress with our tiny team, but certainly there are more resources now than when you were first trying TG2 and 3. In particular Martin's TG getting started videos: http://www.cgscenery.com/basic/
And GeekAtPlay's set of video tutorials:http://geekatplay.com/terragen-tutorials.php
(the Discover Terragen set is available to watch for free on YouTube)
I'm also planning to do a video for Renderosity that will help Terragen Classic users learn how to use Terragen 3/4. With the new ray-traced preview shown earlier in this thread I think working with TG4 will be a lot more pleasant than any previous version of Terragen.
Pricing is not finalized yet, but we are not planning a large price increase, so you can view Terragen 3 pricing as fairly representative.
Kadri, what are your system specs? I've got an overclocked i7 quad core at 4.2Ghz - which is fast but it's a few years old and only 4 cores - and the ray-traced preview works very well for me. It does not use the GPU currently but that may be possible in the future, in which case it could be even faster.