I want to change the video aspect ratio from 16:9 to 4:3.
Have you got a command line to write down?
For mplayer:
mplayer -aspect 16:9
For VLC:
vlc --aspect-ratio 16:9
For Xine (seem to be limited to predefined values):
xine --aspect-ratio <mode> Set aspect ratio of video output. Modes are:
'auto', 'square', '4:3', 'anamorphic', 'dvb'.