Sounds like you are outputting an HD format from your XPression?
If you build your graphics inside a safe title area they won't get cut out in a letterbox or stretch if viewed in 4:3.
You can add a safe title area in the XPression Layout Viewport like this;
In the object manager click on the scene object (very first object in the manger). In the object inspector 'Scene' tab you'll see options for safe title/safe action.
If you change the safe title to left = 20% right = 20% that should give you a 4:3 safe idea.
#XPression