I have both Rex 4 and Rex 3.
I bought Rex 4 because I thought, you know "Hey this obviously must be better than Rex 3, its updated, more optimized and looks a whole lot better!". Oh boy was I wrong....
Here is a screenshot of Rex 3 http://112.imagebam.com/download/NG9h3-yeXurJ4d4l49Ytdg/34872/348715533/fsx%202014-08-24%2017-05-05-27.png
Now here is a screenshot of Rex 4 http://110.imagebam.com/download/AvNLq5vyheL2mkQ2Jlxn4g/34872/348715629/fsx%202014-04-18%2019-14-43-04.png
As you can see Rex 3 looks much more vibrant and alive then Rex 4 (excluding the fact it was raining in Fsx on Rex 4, I have tried it on sunny days but the sky looks so washed out, like someone has painted it on) also as you can see on the left the cloud looks almost 2d. Today (2/09/14) I re-installed Rex 4 again, I did so and all I got was just 2d clouds they were just flat, they were not 3d like the ones in Rex 3, I tried both with Dx9 and Dx10 and they still looked 2d and washed out. I've tried all of Rex 4's settings, Texture, Options etc.
Also which was strange, after that I un-installed Rex 4 and installed Rex 3 again, but for a few hours the clouds looked 2d? I tried to fly on Fsx a few times 5 minutes after installed Rex 3 and they looked 2d, I waited about 3 hours and they looked 3d again? I'm puzzled? Maybe this peculiar thing also happens to Rex 4, maybe I have to wait? I'm so confused...
Even so if the clouds on Rex 4 do turn 3d, I think I'm going to stick with Rex 3 just because of personal preference, because the sim looks so much more vibrant and colourful compared to Rex 4.
Help on the 2d/3d clouds would be appreciate, also leave your opinion on Rex, whether you use Rex 3 or 4, or even use some other type of software.
Here are a few other screenshots from Rex 3