I don't have the full answers but maybe I can shed some light until someone more knowledgeable with the software can fill in the blanks..
The v4 setup works great out of the box after installation. My previous files load properly and with the original v3 settings used. My default settings stayed the same so I did not have to set up the various parameters (lighting, exposure, camera etc etc...) on a new project. You won't even notice too much of a difference... I finished a v3 project and rendered it before switching everything to v4 and was able to continue on with the same project as if nothing changed...
Except for the speed.. dang, I am STILL impressed how much faster this works. My work flow is that once I have finished posing in Poser, and then send it to Octane for rendering, I could get away with about 3 minutes of rendering on a simple 2 or 3 character scene. Though Octane states that the whole render would take 27 minutes to complete, for my work flow, anything above 3 to 5 minutes gives me good enough results... And now with v4, it's cut down quite dramatically and more noticeably is that the scene renders even more smoothly, cleanly in shorter time. I send the saved PNG to Photoshop for post work while the next image is rendering. By the time I'm done with PS the next image is ready to roll out of the renderer.
Not sure about the lighting as I always use the lighting from Red_Specc and their skin shaders. No noticeable changes there..
I think that the default settings for the denoiser (where it is) is set up nicely from the onset... you can notice the difference.
Just to give you an anecdotal idea how much better V4 is, I was looking to increase my video card setup. Currently, I have a 1080 Ti and an original Titan X video cards in my system. I wanted to swap out the Titan for another 1080 Ti but couldn't find one that was affordable (bought my first one for $700 CDN and can't find another one now for less than $1200 CDN... But with the V4 speed improvement, I think I'm good where I am now with my video cards.
I don't think you would have any reasons to go back to v3... v4 is very stable, fast and as always, a joy to work with once you get the basics. I don't use Octane to it's fullest potential by any means, but I couldn't do without now..
A quick thanks to the creator for such a great piece of kit