Sony 70-200 f4G vs f2.8GM: Is G-Master worth +$1,100?

We review Sony's two portrait zooms: the 70-200 f/4G and 70-200 f/2.8 G-Master. The f/4 version of the lens is lighter and WAY less expensive, but how does it compare for sharpness, contrast, and light gathering in low-light?
Buy the 70-200 f4G: sdp.io/s200G
Buy the 70-200 f2.8GM: sdp.io/s200GM