Omid,
The FD 4, not only is the horizontal opening smaller, but the thick vertical post extends closer to center also. So, the opening is quiet a bit smaller / tighter. The FD 4 appears to me to be the same as the Zeiss #4, which is one of my favorite reticles.
The knob that I am referring to replacing is the illumination control / Flash Dot lighting / intensity control. Replacing the sun / moon / S&B logo switch with the Zenith rotary type.
