I don't know the specifics, but X by rights, did not have the coding that is used in newer releases. This is true for most major updates in software.
You can open an X4 "drawing" in X, IF you save the drawing in the X format.
This is done by the Save As.. when you get into the saving dialogue, click on the options button, then you can pick a version of the "X" family you can "Save As.."
Basically saving it in that format.
Hope that helps.
Mike in MN