As I said, having more (or faster) RAM won't really affect render times much, unless you are running into having too little memory. IF you are running into that situation you would know it. Assuming the application doesn't crash from running out of memory, what you get is extremely, extremely slow performance because it's having to use the hard drive/mass storage space as virtual memory, and this is much, much slower than your RAM.
As for how much RAM a particular scene requires, it really depends on a number of factors. If your scene just has clouds and no (or few) objects, then rendering at 4k even with a few v3 layers should be fine in 16GB of RAM. Your comment about number of samples (clouds don't have specific sample numbers anymore, maybe you're talking about the atmosphere?) is a cause for concern because those are very high levels for Terragen 4 (which comes with Defer Atmo enabled by default, which means fewer atmo samples are required), but higher samples should not require significantly higher RAM as far as I know.
So basically, 16GB should be OK for most purposes for now, but RAM is fairly cheap so upgrade to 32GB when you can if you're serious about really complex scenes with lots of v3 cloud layers as well as complex objects/populations.