I think the render size is the problem. You need RAM to store the image while rendering, and 4k is quite big. Yoe could try rendering the image in parts and putting them together afterwards. There is a script that does it for you, but I have heard it may be no longer working - I could be wrong, best to check it out for yourself:- http://www.macouno.com/2010/03/18/really-big-render/
An alternative is to split your scene into layers and render each layer separately, then composite the layers.
I would disagree that 4K is large. I have done 100s of renders at that size. It seems to be the combination of the size AND SSS that is causing the problem here. But I still can’t imagine how a simple scene would use up 2 GB of memory!
Thanks. I have downloaded and run the Python script on my scene by splitting it up to a 2x2 formation.
Unfortunately, in the areas where you join the split images together, there is some variation in lighting (which I can’t explain).
I was able to workaround this by executing the Python script in a 3x3 formation and then combining these renders with the first set of renders.