I think I prefer the bottom-right, 3/2/4, if you look into the distance at the very far plateau, the distant lighting in 4/2/4 seems flatter, over-riding more of the slightly shadowed areas of the cliff and seemingly covering the entire surface in GI, regardless of primary shading/displacements on the rock face. Maybe 4/2/4 is more 'correct' but I agree that 2/2/4 is pretty damn fine. Is it worth the nearly 50% extra render time to go for 3/2/4? I'd say, in this case, no.
I absolutely love the GI options now, we can render out final scenes at respectable, sensible, render quality but with as high(or low) GI as you like, mega-sampling the prepass at render detail/1 and then rendering at 0.5 - 0.75 detail for the final pass using that GI cache. Or, go the opposite way to save on time, using a lower GI cache for speed. And the ability to use cache blending for animation is fantastic.
Also, on upping GI detail further, maxing the 'ray detail multiplier' removes the triangulation of the initial GI pass, giving a more accurate spread of light to eventually use in rendering the last pass, before tuning back the renderer(and RDM) for the final pass, giving much better results, even at GI relative detail/1.
Cool tests, Martin.