Thanks, Oshyan...and you got me thinking, so I tried a few tests that have given me a clue to what may be my issue (emphasis on 'my'). I'll consider sending the whole scene, but I'm a little reluctant to show it all at the moment since it's still a work in progress...however, I'll explain what I've found after your last note.
As of now, I think it's related for the most part to a couple of populations of grasses that I made when I was creating the desert initially. It is a huge area of desert terrain populated with bushes of different sizes, pine trees on mountains about 5 miles away in the background, and two different grasses. I think it's the number of grass objects in those populations that may be too much to handle initially...here's why:
I opened the scene and disabled everything initially, except for some basic shaders. Then one by one I enabled things like the terrain and atmosphere without any real problems. I left the objects for last, then enabling-disabling as I checked them one by one, each time trying RTP. With the bushes and pine tree type objects enabled, the RTP worked quickly enough and I could start a render fast enough to be okay with it. But when I enabled the first grass population (no other objects enabled) I saw the first big delay as the preview went black, and things seemed to not respond and freeze--but after several minutes it finally showed the RTP. Same thing when I tested the second grass population. Ah..so..
I then did a quick test with just the bush populations one by one, and while each would load up fairly quickly, with the RTP going, it was very clear that the higher the number in the population, the more it too for RTP to crunch the numbers, so to speak. As of now, my guess is that it isn't actually the size of the object or how large the area that population covers, but the density of the population (which I suppose means too many individual objects over too much area = just too much to handle with RTP enabled.)
I've just tested enabling all the bushes, trees, cacti, etc... --but not the grasses-- and RTP will load it all (in due time) and though it may take 10+ minutes for a render to actually show progress, the render does finally begin. But if I have everything enabled, grasses included, the whole system may eventually crash, if I can't get enough control to open Task Manager and terminate it. Had to press 'reset' once yesterday when the screen went dark. Oh, I might add that this testing was on my slower machine, so that is a factor.
To stop for now, I'd say that as long as I Resist Temptation to Press the RTP button in this scene, I'm probably okay in T4. I'm going to try turning RTP off in the preview and then hit Render just out of curiosity to see if the render starts more quickly if RTP isn't on in the preview. Will report back.