This is typically a MAME setting, not a Maximus Arcade one.
Set keepaspect 1 in your mame.ini
Or add -keepaspect to your command line in the MAME config section of Maximus Arcade preferences.
However, if the actual resolution that MAME is putting out is really 1024x768 (which is already 4:3), either your monitor is set to stretch, or your video card is configured to scale to full screen instead of scale to aspect.