Add the below details to your XF86Config-4 file. The XF86Config file is far
from complete and will not work as is. You should check the first ~10 or so lines of your /var/log/XFree86.0.log for which config file is in use.... it explicitly tells you which was used to launch the X server.
Virtual 1600 1200
Modes "1600x1200" "1280x1024" ## add more desired modes here ##