Tertiary Gain

OK, I hear you cry, why Tertiary? Whatever happened to Secondary Gain?
Well there is one, but this is just a much better puzzle. It has a level of 9 (5.1.1.2) which is trickier than Primary Gain and is definitely approaching the top end of the range for a 4x4x4 cube. The completed cube is 1.6cm on each side.