4x3 is roughly the size of the window I had between the control & tracking room in my old studio. I rarely wished it was any bigger. I used 2 panes of automotive windshield glass separated by several inches, with the control room pane angled towards the tracking room pane (maybe 6 inches of separation at the bottom and 3 inches at the top).
Windshield glass is a sandwich consisting of 2 thin panes separated by a clear plastic sheet. I used it on a hunch, but it seemed to work very well. I never measured the attenuation, but it was pretty high. Once you closed the single steel door between the 2 rooms, you could easily hold a pleasant conversation even with a loud rock band playing in the tracking room. As you would expect, most of what got through was in the lowest octave or two.