Reduction options control how radical the encoder is in throwing away apparently poor candidate estimates during motion estimation. A setting of 1 means very few blocks are discarded early which makes for slow encoding but quality as good as it gets. A setting of 4 makes for fast encoding but can impact quality. The 4x4 option controls discarding during the initial 4*4 sub-sampled search stage, the 2x2 option controls discarding during the secondary 2*2 sub-sampled stage.
