If you are going to just view something on your monitor, stick with square pixels, because the monitor has square pixels.
1900x1200 is not a 4:3 ratio but that almost certainly isn't framed in a 4:3 area rectangle. If you measured it you'd find it to be about 1.58 to 1
You can render at 1024x768 but that will take longer than 640x480.
If you want to see a 640x480 movie larger, the easiest thing to do is full screen the player or use Quicktime's View>Present Movie option.