I tried to adjust the 4:3 aspect ratio (1920x1440) on my 16:9 monitor (native 2560x1440). It doesn't work, the picture gets stretched over the screen. I can force 1:1 picture display with my monitor, but I use the 4:3 ratio with many older games (Ghost Recon, Raven Shield, Unreal, UT, AvP2, etc) because there are problems with the field of view on 16:9. The games auto adjust the ratio on my system with black bars at the sides. This should work in ArmA3, too.

A menu overlay glitch appears when I switch back to 16:9, the overlays and menu screen keep the ratio 4:3. {F18336} {F18337}